One of the good things to come out of the Iran-contra hearings was the phrase "plausible deniability" used by the White House minions. The object of plausible deniability was to give the president an out if anyone attempted to trace dirty White House linen to the Oval Office.

"The president didn't know because I didn't tell him," bragged Adm. Poindexter during the hearings, thus giving Mr. Reagan the deniability he so badly needed during the worst period of his term.

It is also believed that other witnesses provided themselves with plausible deniability while cooking up various covert actions, but each witness was judged differently. The public decided in the case of Albert Hakim that his deniability was a work of treachery, while in the case of Ollie North it was an act of patriotism.

Although plausible deniability became fashionable during the Iran-contra hearings, it is really as old as American apple pie, and there isn't a segment of our society that doesn't use it.

For example, suppose you are waiting on a check from a company. This is the kind of plausible deniability the firm provides: "We're sorry but Mr. Wigwam, who signs our checks, is in the hospital and won't be back for three months." Or "We regret our computer is down and we will not be able to deal with the problem until we hear from our programmer in Tokyo."

Tradesmen all carry plausible deniability cards in their wallets, which they take out and recite at will. "I didn't promise to deliver the slipcovers in August -- I told you I would have them ready between August and November. It's written right here in my pocket calendar." That or "We were there yesterday but no one answered the door."

Personnel managers are experts in plausible deniability. "There must be some mistake. We never offered you a job -- we just told you that if something opened up you would get equal consideration. If you don't accept this as a reasonable explanation we will shred all copies of your re'sume'."

Even schoolchildren learn early in life to provide themselves with deniability. "I didn't know I was supposed to do homework because nobody told me." Or "Miss Mathers said I didn't have to turn in my term paper until Tuesday. I have the date marked in my assignment book, so she must have gotten her instructions all wrong."

And while we're at it, let's not forget the P.D. practiced by teen-agers. "The reason I didn't bring the car home last night is it broke down and I could not find anyone to fix it, and I didn't want to leave it sitting out there on the Beltway, so I guarded it all night long."

"What proof do you have?"

"Penny sat up with me so I wouldn't get cold."

Plausible deniability is the most important kind of deniability there is. It is a story that can't be shaken whether told by an airline clerk who tells you you have no reservation or an auto mechanic who claims he has repaired your brakes. No one in the country could live without it. Even in affairs of the heart P.D. plays a most vital role. Who in this great country has not been caught going out with somebody while he or she belongs to another, and in defense uses the ultimate excuse, "I was only following orders."