Ask a stupid question, the adage goes, and get a stupid answer. But that's not necessarily so with people in high places. With the right air of ignorance or innocence, you may get, to a stupid-sounding question, an illuminating answer.

Walter Lippmann, legend has it, often carried this technique to the point of sitting back, arms akimbo, and asking no question at all. As the silence became unbearable, even strong statesmen would crack and begin to babble wonderfully rewarding revelations.

But no part of this principle applies to a question from which, realistically, no answer could be expected that would enrich public understanding-- and to which almost any conceivable answer would be likely to mislead. A vivid, current case in point: the question of whether the Reagan administration is engaging, or planning to engage, in "covert activity."

As variously put to the president at his last news conference: "Mr. President, have you approved of covert activity to destabilize the present government of Nicaragua?" Or: "Is it possible that you can tell us that there is no secret plan . . . to surreptitiously involve Americans" in Latin America in a "covert manner"?

Now these are legitimate concerns; they derive from persistent reports that the Reagan administration is working up a "covert" scheme to "destabilize" the Sandinista regime in Nicaragua with the aid of surrogate Argentinian "paramilitary" forces. And yet, when such concerns take the form of public questioning, the mind reels.>

A "glossary" of CIA jargon included in the 1976 report of the Senate's select committee on U.S. intelligence operations (the Church committee) defines "covert operations" as those "planned and executed against foreign governments, installations and individuals so as to conceal the identity of the sponsor or else to permit the sponsor's plausible denial. . . . " The word "covert," says Webster's, means "concealed, hidden, disguised."

So the president was being asked to reveal what is required, by definition, to be hidden and to discuss what is by its very nature unmentionable. And while this may owe something to the time-honored "dumb question" technique, it probably owes more to a genuine conflict in the American public's attitude toward that gray area between diplomacy and war--a lingering reluctance to accept the utility, and still less the morality, of "dirty tricks."

The Church committee's voluminous hearings (complete with horror stories of clandestine assassinations, secret subversion, unacknowledged paramilitary campaigns) had a profound impact. But subsequent regulations and legislative reforms were thought to have eased public anxieties. Covert activities were authorized only as an extraordinary last resort, under executive safeguards and with strict congressional oversight.

But clearly, what had seemed to be settled remains very much unsettled in the public mind. And one reason for this is that as a practical political matter the Reagan administration is no better able than its predecessors to address the issue head-on.

If true confessions are out, so, as a practical matter, are "plausible denials." For one thing, leaks can quickly make them implausible. For another, there is something to be said for the answer the president actually gave: that the foreclosing of "options" is tactically unsound.

But refusal to rule out anything has the effect of ruling in everything--as Harry Truman discovered when he was asked whether he was "considering" using atomic weapons in Korea and replied to the effect that he was considering everything. It took days to reassure American allies.

Even "no comment" has its pitfalls. Said Sam Donaldson, on ABC's "Nightline," right after the Reagan press conference: "By refusing to comment, the president knows he's leaving the impression that activities to overthrow other governments may already be under way--an impression perhaps deliberately meant to worry America's foreign adversaries, but one that also seems to worry a lot of American citizens as well."

That's a fairly large leap to a heavy and quite possibly misleading conclusion. But it's an inevitable leap for a lot of Americans, absent a high level of confidence in the president's Central American policy in general, and the new executive and congressional strictures on "covert activities" in particular.