What Lisa Birnbach started for laughs is now beyond the joke stage. It is deep into the craze stage. It is "Prep." As anyone who has been small-gift shopping lately knows, les choses prep overflow the store shelves like cutesy-poo junk food aimed at the collective insecurity of adolescent America. There is an entire, fully licensed and franchised cottage industry that has grown up around the best-selling "Official Preppy Handbook," including preppy T-shirts ($12.50 per), preppy aprons ($12), preppy tote bags ($10) and book bags ($10), and preppy stationery kits ($7.50), desk diary ($5.95) and even boxed Christmas cards ($6). All, of course, in preppy plaid. Preppiness to the right, preppiness to the left, Heavens to Elliot Richardson, it's enough of an overload to make a true preppy say, "Oh gross, I think I may barf."

At the unashamed, outrageous, go-for-broke, hot-hot-pink and hubba-hubba, electric-wild-lime-green core of this preppiness is the editor of the handbook, 25-year-old Lisa Birnbach, who has been appearing on television lately as the Emily Post of Prep. What she has done -- with considerable skill and presumably at considerable profit -- is to identify and satirically codify the life style of our ruling class. Depending upon your bank balance and self-image, she has either celebrated Wasp America, or hung it out to dry on its own L.L. Bean clothesline. Consider these excerpts:

* ". . . Mummy and Daddy have carefully selected first and middle names (at least one of each) for the newborn that correspond to the names of dormitories at their Prep alma maters . . ."

* ". . . I gave birth to a baby girl . . . She flatly refused Pampers and demanded 100-percent cotton diapers clasped with a gold kilt pin . . . She didn't play 'house,' she played 'cottage.' Instead of playing 'store,' she favored 'investment banking.' . . ."

* ". . . Charge at any store, whether or not you have an account. Simply give them your name and address in the most disinterested manner possible . . ."

* ". . . Preppies wear clothes for 25 years and no one can tell the difference. The fabrics, the cuts, the colors are the same, year after year after year . . ."

* ". . . You are involved in a rear-end collision on I-95. You leave your car and . . . answer Discover that your law partner skippered other driver's older brother's boat in Bermuda Race of '55, and go out for a drink . . ."

Birnbach says that she herself was a preppy, is a preppy and always will be a preppy. ("Don't I know you from somewhere? Did you go to Riverdale Country Day School?") So, if this is a satire, then she is satirizing herself. How swell. How witty. How wise of her to affirm that she has done this in the name of populism. ("I believe in a true democracy everyone can become upper class . . . Anyone who wants to be a preppy can be.") What she promises is the birthright just by following a few simple rules, as if parentage and ethnicity don't have a thing to do with it -- as if it were buyable. Such a nouveau concept. And such a delicious irony that Lisa Birnbach, now the Avatar of Waspdom, is the daughter of a refugee from Nazi Germany, a Jewish girl from New York City, a nouveau herself.

So how much, a person might reasonably ask, does Lisa Birnbach actually know about these preppies? About their pain? About their angst? About their waxy yellow buildup?

These questions make her nervous.

"Most everybody considers me fluff," she says. (Fluff? Her? Someone who sets the tone for a conversation by saying, "Oh God, I haven't combed my hair for at least two hours?") "People just expect me to be a performer. Someone to say, 'Hey, great to see you, Merv, haven't seen you since Vegas.' They say to me, 'Tell me about loafers, Lisa.'. . . It's not that I don't want to be taken seriously, but I've been more comfortable telling them about loafers because that's not about me, and it's an easy way to make them laugh."

The subject was preppies, wasn't it? Real preppies?

"Well, we're talking about the elite. George Bush is a luminary we all are following. Even better, George Plimpton! He can't even hold down a job . . ."

She looks for laughs, finds none, sighs, switches gears.

". . . Okay, I'll tell you as much as I know. I'm still fringe. I'm not as in as them."

(Truth: In the peeling of the onion, the most profound tears come half the way down, with the realization that one can be on the inside but not even near the core.)

"As far as I know, they're interested in keeping the status quo as much as possible. They want their kids to intermarry other preppies. They want to make sure of the zoning laws in Connecticut so they can buy liquor in bulk when they have a party; they drink a lot . . . Appearances are very important. Credentials are very important. They own several crew-neck sweaters. All wool . . . I don't know of their angst. I don't know if they know of their angst. They go crazy when their apartment building switches from manual elevators to self-service; I've seen that happen. They worry that they could lose their jobs if Minute Maid goes to another agency . . ."

But real pain? Surely even they have real pain?

It is a serious question, and it gets a serious answer.

"Yes. We get it from Updike and Cheever, we get their pain. Pain in watching their children leave home, or turn into black sheep. Pain in watching other people, different people, join their world . . . They have the pain we all have, but what sets them apart from the rest of us is that they don't show it. They don't show pain. Don't show concern. They are absolutely confident in their own ability to pull it off, no matter what it is."

No fears?

"No, they have fears. Big fears. They're terrified just like the rest of us . . ."

She can't resist.

". . . Only they worry about their top spin, not whether Crest will be available in 1995. They aren't different -- it just looks better on them."

Despite her position, despite her diploma, even despite her pearls, Birnbach doesn't seem like a genuine preppy. ("I am a preppy, but I'm part of the bohemian subculture.") How can someone on the "us" side of "us and them" be a real preppy? A Pepper maybe, but not a preppy. She is a nouveau preppy. She's got a past, but not a history.

". . . You're giving me a hard time . . . You want me to apologize, and I have nothing to apologize for . . . I don't see myself as an important figure. I had no idea the book would do so well. No idea. I'm this kid that did a book -- that's all . . . It's very scary that I could be getting this much attention, that the BBC calls and asks my opinion on social matters. It's totally out of control . . . You fantasize about fame, but you never really see it coming. I read bylines in newspapers. I thought at best someone would read the name Lisa Birnbach, and know it like I know R.W. Apple Jr. I feel, in a way, too young for this -- not ready for this. I don't have my hand on the pulse of this country. Maybe I'm witty, but I'm not brilliant . . ."

There is a calculated pause.

". . . On the other hand I'm very ambitious, and I certainly do want to cash in . . ."

Give the woman credit for the pre-op set up and the post-op giggle. But there is still the question of the mixed signal: Birnbach, who talks about being "pigeonholed as a preppy," is getting her kicks out on the college lecture circuit ("At Georgetown I even sang 'Stop in the Name of Love' ") and the book-hype tour for the new Preppy Diary. She went to Barnard and Brown (as proletarian as it gets in the Ivies), then worked at a Mad Ave. ad agency and The Village Voice -- liberal, intellectual credentials. Yet try to press through the cosmetology and get to the satire of the Handbook, and she kisses it off with a stock line like, "I decided when I was 17 I'd never take myself seriously."

It's rather interesting, this battle within Lisa Birnbach, her ego ("I think it's a funny book, and more than funny, it's a smart book") versus her conscience ("I'm sure there's anger behind it; I don't like a lot of preppies; I don't think I've dated a preppy in years"). And in her case, if one has to choose between ego and conscience, after reading her true confession it should be no contest:

"When I was at Brown, I was sort of taken in by a circle of hard-core preppies. It was a very heady experience. You don't have to have a lot of money to be a preppy, but they did. And they had that air about them, that undeniable confidence, that breeding. They'd do things on a whim. They'd say, 'Let's go to the Cape for the weekend.' And we'd pick up and go. It was like they had no responsibility to anything but their own good times, like they knew no matter what, they were set and nothing could touch them. We had fun, I mean fun with a capital F. But one day I was sitting by myself and I realized -- Not one of them really knows me. Not one of them really cares at all about me. I asked myself what exactly was going on with me and them, and I knew that nothing was. Not a damned thing."

At last count "The Official Preppy Handbook" was into its 23rd printing and had sold 1,243,723 copies, and Lisa Birnbach had been on "Today" three times, talking about loafers.