Brother, can you spare a trillion?

Once upon a time, and not so very long ago, the word ''trillion'' was always used facetiously. Most everyone thought of it as a sort of make-believe word, like zillion, which was uttered to quantify something beyond all belief. ''I've got a zillion marbles'' was more or less the same as ''I've got a trillion marbles.''

Many people alive today can remember the time when only astronomers would use trillion seriously. Occasionally they might employ other silly words in a serious way, like decillion, but they were talking bout light years, or the speed of a comet, or something like that which didn't affect your weekend in the country. Even the planet Pluto isn't that far away.

If you did happen to read trillion aloud to your spouse, it was in a jocund manner, dealing with the distance of a star, and you would get the expected response of a clicking tongue.

But no one used a word like trillion talking about money.

However, Washington, the Disneyland of the news media, has changed all that overnight. Trillion is an acceptable word now. It is the only word which can be used to describe a certain situation. In this case the size of America's national debt.

When our man in Washington mentioned that the deficit was a trillion dollars, we smiled and said, ''You're kidding. Trillion is something with at least a dozen zeros after it.''

''You're thinking of decillion,'' he said, ''which has 33 zeros. Trillion is a much smaller number with only 12 zeros.'' Well, we happen to think the number 12 sounds big, even if it's only a shoe size.

Just to be clear about it all, we asked a local financial expert how the United States could pay off the debt, and it seems it borrows on the money. In fact it has to borrow money to pay interest on the money it borrows. Try that on your hometown bank for yourself sometime if you want to hear a stone-faced banker go into hysterics.

''Interest on the national debt will jump 12.7 percent to $144.5 billion in fiscal 1984,'' he said. ''So I think the government will have to borrow $103.2 billion as a net interest outlay. Does that answer your question?''

It doesn't answer the question, but it sure settles any desire about asking any more.

The next word is quadrillion. Does anyone ever say quadrillion seriously?