You can't follow the debate over the Reagan administration's Central America policy, and still less can you appreciate its ultimate absurdity, without a dictionary. The operative word is "covert." Webster's New World Dictionary is downright adamant about its meaning: "Concealed, hidden, disguised, or surreptitious."

That's the point of "covert operations": the U.S. hand is supposed to be hidden from the American public and other interested parties; American involvement is supposed to be convincingly deniable in case somebody takes offense; that way, you avoid the awkwardness of being seen violating treaties or breaking domestic and/or international law--and the embarrassment if the operation fails.

I go through this slowly because it all sounds so plausible ("Congress Debates Secret War in Nicaragua") that it takes time for the inanity of it to sink in. Once it does, the futility of constructive discussion begins to sink in. And that's what's happening now in the argument in Congress over Ronald Reagan's plan to ease the heat on El Salvador by lending "covert" support to anti-government forces in Nicaragua.

By reason of its "covert" nature, the idea is being talked to death. The process is worth examining, step by inexorable step.

Leave aside whether "covert" activity of any kind to destabilize unfriendly governments is consistent with American values--or workable. You could make some case for the administration scheme on its terms a year ago; the idea was that support for opposition elements in Nicaragua would help interdict Nicaraguan support for the rebels in El Salvador, either directly or indirectly by making the Sandinista government pay a price.

Congressional intelligence oversight committees had given secret concurrence, with the proviso that the purpose not be to overthrow the government in Managua. But last fall, the effort suddenly ceased to be "concealed, hidden, disguised." It was all over the press, unpersuasively denied by the U.S. government, confirmed by the Nicaraguan opposition forces.

So total was the breakdown of secrecy that Congress made the secret proviso public in the form of the so-called Boland amendment. Recently Ronald Reagan himself gave away the plot and his real purpose:

If Congress wants "to tell us that we can give money and do the same things we've been doing . . . providing subsistence and so forth to these people directly and making it overt instead of covert, that's all right with me," he told a small group of reporters in his office. But not, he added, if the administration "would have to enforce restrictions on the (Nicaraguan) freedom fighters as to what tactics they would use." In other words, no nice distinctions about the opposition's purposes between "interdiction" of supply lines to El Salvador and overthrowing the Sandinista junta.

Congress isn't going to be foolish enough to license the administration to jump in openly on the side of a Nicaraguan insurrection. So the question is whether the administration can persuade Congress not to vote a total ban on continued "covert" U.S. activity in Nicaragua. That's where the whole debate goes off the rails.

An outright ban would have a precedent: the Clark amendment forbidding covert U.S. activity in Angola in 1975. But complications set in if Congress actually votes to reject a ban. That becomes a go-ahead, publicly, to conduct a "covert" operation. As one congressional critic puts it, "By not saying no, we're saying yes."

For just this reason, there is growing demand among younger members of the House, in particular, to go on record to cut off any "covert" intervention in Nicaragua. The leadership until recently has taken the traditional line that any such interference with the president's foreign policy prerogatives could be turned into a "Who Lost China?" campaign talking-point if El Salvador is lost.

But support for the commander in chief is one thing; open approval, even indirectly, of "dirty tricks" is quite a different political issue, post-Vietnam. It is reliably reported that so influential a figure as Majority Leader Jim Wright, who spoke up eloquently in support of the president after his address to a joint session, was even more eloquent in a recent closed-door meeting of the House Foreign Affairs Committee in favor of shutting down "covert" operations in Nicaragua.

The cost of such a compromise for those in Congress who like nothing about the administration's Central America policy will probably be a vote for most of the military aid money for El Salvador--and extra funds for overt efforts to shut off the Nicaraguan supply lines through Honduras. That may not be enough to meet Ronald Reagn's purposes. But it might help us all think more clearly about the implications, in the conduct of U.S. policy, of the word "covert."