HAVING DEFENDED that ubiquitous but unpopular conversational H opener, the inquiry about a new acquaintance's occupation, Miss Manners will now admit that some of the deadliest conversation she knows follows the question, "What do you do?"

When people have objected to being so queried by those whom they have just met, Miss Manners has always made the lackluster defense that conversations have to start somewhere, and a person's job is likely to produce a clue as to his or her interests (or antipathies) on which initial chitchat can be based. It is at least better to begin with one's field than with probings about one's financial status or family connections, she has argued rather spiritlessly.

But the evidence is piling up that all remarks following an honest answer to such a question fall into one of four categories, each worse than the one before.

There is, of course, the social request for free labor, whether advice or even services. Everyone knows this is hideously gauche, and yet people continue to make such demands. Miss Manners has heard a number of strategies from the recipients to discourage this, beginning with her Great Uncle Simon who used to reply, when asked at parties for his medical opinion, "Certainly. Please take off your clothes."

Then there are the complaints. Miss Manners once saw the president of an international chain of household equipment stores furiously attacked at a black tie party by a horde of well-dressed people whose refrigerators were on the blink or whose vacuum cleaners didn't -- uh, work. She would have been shocked if she had not been so anxious to explain that the missing tassles on her Austrian window shades had never arrived.

Artists of various kinds are among those plagued with the next sort of conversation, the one designed to find out how well one is actually doing in one's chosen profession. "Haven't you finished that book yet?" is apparently considered charming conversation to make to an author, and "How many paintings did you sell?" to an artist.

But the worst of all is the smart remark, based on a primitive understanding of what people in a particular field actually do, and designed to twit them about it. A variation assumes that the other person goes about flexing his expertise at the expense of everyone he meets.

Thus, a military person is asked whether he is yearning for war in order to destroy people; a teacher is told, "I suppose you're going to notice if I make any mistakes"; a psychiatrist is invited to guess who is normal and what they are thinking; and a reporter is told, after each inane remark, "I suppose you're going to put that into the paper -- I better watch out."

Miss Manners can only advise the targets to be tolerant and patient, knowing that the will to do so wears thin along about the 128th time each remark is repeated. She cannot put a ban on clever comebacks but does make a plea that they be reasonably good-natured. One of her favorites was developed by a geneticist who is often asked, as many scientists are, whether he was going to start making people.

"Oh, I already have," he replies. "A boy and a girl."

Etiquette advisers are not immune, and Miss Manners admits that her smile has grown very weak indeed when people say, "Oh, I guess I better watch myself or you'll catch me doing something wrong," as if she were in the habit of giving out traffic tickets for lunch table violations.

So she asks the help of her Gentle Readers in telling her what the inevitable remark is, upon the announcement of their various occupations, so that she may compile a directory of clever things to refrain from saying to people after they have replied to "What do you do?" MISS MANNERS RESPONDS

Q. My husband and I have a 13-year-old "typical teen-ager," who is very concerned about appearance. However, she enjoys having very long fingernails, always manicured and polished. I object to their length because of their appearance being that of an older woman -- a harlot, I should say.

Being a nurse, I also consider them a carrier of bacteria. None of her friends has such talons, and her nails are a topic of conversation (and envy) among them.

My husband does not support me in my effort to trim the situation. Any suggestions?

A. Miss Manners' first suggestion was to find something more significant to fight about. It is not her impression that growing fingernails is the worst behavior that typical teen-agers can invent, if pressed.

After thinking it over, Miss Manners decided to discard this suggestion. Why force the child to think up something else that will annoy you?

It was Miss Manners' dear mother, a teacher, who first pointed out to her the harm of complete parental tolerance. During the 1960s, when many parents not only condoned but imitated their children's attempts to rebel, the poor children were driven mad trying to find something that would work the age-old trick of shocking their elders. She predicted that many would find nothing left to them but to indulge in extreme forms of religion and chastity, if they wanted to earn the disapproval of their parents.

It is with this in mind that Miss Manners has decided that there would be little harm in your fighting your daughter over the ludicrous question of the length of her fingernails. It should keep you both happy.

Q. I am in my late twenties and am about to enter my second marriage. My family lives in another state and will be traveling here for the wedding, as this is now my home, and my fiance and I are providing most of the financing for the wedding.

My parents were divorced about 10 years ago and my father married the Other Woman, whom I'll call O. There is still a lot of tension between my mother and O., yet I know that they will be civil to each other for the sake of my wedding.

The problem is that we are planning a part of the ceremony in which we acknowledge our families and say something special about them and the way we feel about their roles in our growth, and that sort of thing.

I like O. well enough, and I don't blame her for my parents' split; nonetheless, I don't feel special about her and I don't consider her family. On the other hand, I feel awkward about having her and my father sit apart, partly because one does not separate a married couple and partly because everyone would wonder why they weren't sitting together. Can you suggest a course of action that would solve the etiquette problem and still allow me to be true to my feelings about my family?

A. Yes, but you won't follow it.

Miss Manners does not believe that a public ceremony is the place to discuss the roles of individual family members in your growth, in however edited and favorable a manner. (If you were to do it thoroughly and honestly, you would probably have to admit that your first husband played a large role, for better or worse, in your growth.)

Why can't you write long, sentimental letters to each of your parents, expressing your gratitude? Or if you wish to do it in front of your and their friends, you could put it all into toasts at the wedding reception. It seems to Miss Manners that the promises that you and your bridegroom make to each other in the ceremony are quite enough in the way of publicly announced intimacy for that part of the occasion.

Knowing that you are going to go ahead anyway with your plan, Miss Manners will now assure you that it is quite common and natural for a bride to focus on her original parents, rather than on subsequent additions. It is civilized of your mother and stepmother to call a truce during your wedding, and no violations of anyone's rights for you to treat your mother as the sole mother present and your father as the sole father. No one will think, if your parents sit together, that they are rekindling their relationship at the expense of his wife.

Nor would it be improper for your original parents to sit apart, your father with his second wife, and for you nevertheless to single them out where they are. Copyright (c) 1982, United Feature Syndicate, Inc.