In social life it is called etiquette. In personal life it is called hypocrisy. In political life it is called diplomacy. Americans tactlessly persist in calling it by its generic name: lying. Americans have a tolerance for many things. Lying in public is not one of them.

Take last week. From all the handwringing, you could be forgiven for thinking that the worst transgression in the Achille Lauro affair was not that terrorists shot an old man or that Italy let the ringleader go, but that for eight hours Egypt's President Mubarak lied about the whereabouts of the terrorists. For this misdemeanor, the Egyptian ambassador was subjected to a cross- examination on "Nightline" of the sort not seen since some hapless witness decided to perjure himself before Edward Bennett Williams. (Certainly nothing of the sort was encountered by Yasser Arafat on the same show.)

Caught in a lie, the ambassador tried to slip away. Alas, he could run but he couldn't hide. Mubarak, on the other hand, took the first opportunity to admit to "a diplomatic deception." Rather than regret, he evinced surprise that anyone should have taken much notice, let alone offense. After all, he has more important things to worry about than passing the Watergate truth test. For starters: the stability of his regime, on which hangs the security of 46 million people. Caught between fanatic Islamic fundamentalists, on the one hand, and angry American allies, on the other, he decided that the better part of valor was an eight-hour lie. Big deal.

But for Americans, famous for their frankness, and not yet jaundiced by centuries of statecraft, it is a big deal. It has been known since 1604 that an ambassador is an honest man sent abroad to lie for the commonwealth. Yet after two centuries at the game, Americans have yet to get used to the idea. We have the contrary view that in diplomacy truth- telling is always a virtue. It is a charming and expensive indulgence that only a young country can believe and only a big country can afford.

Not that the United States has not told some whoppers. There was U-2 lie. Thinking the pilot dead and the plane destroyed, the Eisenhower State Department put out the story that the U-2 was an off-course weather plane. And there was Adlai Stevenson's lie about the Bay of Pigs. Lied to by the CIA, he told the Security Council that the United States had nothing to do with the invasion. After Vietnam and Watergate, native American revulsion with political lying peaked: in 1976 a president was elected on a platform of truth- telling and little else.

But lying can be so indispensable that not even President Carter could do without it. In advance of the Iran rescue mission, Jody Powell planned a cover story (about a possible blockade), and, when Jack Nelson of the Los Angeles Times got wind of a mission, Powell used it and categorically denied any plans for a rescue. Even then there were some who carped that Powell should have issued a "no comment" rather than a denial, so as not to allow a true lie to pass his lips. Of course, a "no comment" would have aroused suspicions and jeopardized lives. For some reporters, however, jeopardizing credibility is the greater sin. They said Powell should resign.

Now the good news. We may be learning to relax. A note of maturity has issued from, as usual, State Department philosopher George Shultz. Earlier this year, he was asked about alleged CIA involvement in a bombing in Beirut. "If the CIA denies something, it's denied," he replied. Note: not "false," but "denied." Lovely. "Denied": a category all its own, hovering somewhere between truth and falsity and, for countries prepared to act in the world, indispensable. Whether or not Shultz was being intentionally wry, he had delivered a lesson in old world statecraft.

The notion of deniability, like old world statecraft, is foreign to American sensibilities. Nevertheless, deniability is quite valuable to American diplomacy. Why, for example, is so much Reagan Doctrine aid to anticommunist guerrillas "covert"? The term seems both ludicrous (Can't everyone read about it in The Washington Post?) and sinister (Is the government trying to hide something from the electorate?). In fact, the major purpose of "secret" aid to, say, Afghan guerrillas is not to hide the facts from Americans (or Russians, for that matter: they subscribe to The Post, too), but to provide protective cover to our allies. Pakistan funnels our aid to the Afghan insurgents, but is too vulnerable to Soviet pressure to declare so openly. Moscow knows what is going on, of course, but for Pakistan to announce it would be nothing more than a provocation. Why add insult to insurgency? So all parties agree to a fiction.

Fiction, a high form of literature, is considered, in this country at least, a low form of diplomacy. Diplomacy being the means of advancing the interests of one's country by means short of war, it is hard to see why this should be so.

I concede that truth is preferable. For one thing it is easier to memorize. On the other hand, it can be habit forming. What to do? Graham Greene had it right. "He always preferred the truth," he says of his hero in "The Human Factor." "Except on really important occasions."