A TERRIBLE THING happened in Washington last Monday. A U.S. State Department diplomat lost his temper and cussed in front of the Iranian charge d'affaires.

This is what happened. The charge d'affaires, Ali Agah, had been called in by State to be told that he and his staff were being expelled from the United States after five months of diddling around trying to resolve the hostage question.

According to Mr. Agah, Henry Precht, in charge of the U.S. State Department's Iranian desk, exploded in front of the charge d'affaires with the word "Bulls---."

Mr. Agah was shocked that anyone would talk to an Iranian diplomat in such terms. He stomped out of the meeting and in one of the most moving statements ever made on the steps of the Iranian Embassy, Agah said, "We will not take any longer to have any of my brothers insulted," (sic) as tears poured down the checks of the reporters who surrounded him. "The U.S. government does not understand us. They do not understand our revolution. They use language that I am ashamed of. The revolution gave us dignity and by insulting us they are trying to take our dignity away."

The State Department will obviously have to take strong measures against Mr. Precht for insulting an Iranian diplomat. How can you have good relations with a country holding 50 of your diplomats as hostages for almost six months if a U.S. government official tells a charge d'affaires that what he is saying is "bulls---"?

The worst thing you can do to an Iranian diplomat is take away his dignity.

Just because Iran has a flaky ayatollah running the country, and a bunch of thugs occupying the U.S. Embassy in Tehran, and a government made up of lying mullahs, is no reason for a State Department official to cuss out a representative of that country.

Precht hurt Agah's feelings as well as that of Mohammad Lavassani, the other Iranian official in attendance. In their wildest dreams they never imagined that anyone would ever use such a strong expletive in their presence.

But the damage has been done and both Agah and Lavassani have left the country in dismay. The big question is how we can make it up to them. An apology by State Department spokesman Hodding Carter would be a first step. The second would be a public humiliation of Mr. Precht in the courtyard of the department's headquarters, with the entire U.S. diplomatic corps in attendance.

At a formal parade Secretary of State Vance would strip the leather straps off Precht's briefcase and wash out his mouth with soap and water.

If this didn't satisfy the Iranians, President Carter could go on national television and announce that he was forbidding the use of the word by anyone in his administration unless it was first cleared with him.

The most important thing we have learned from this affair is that when you take away a diplomat's dignity by cussing him, you not only insult him but also the country he represents. Precht was not only saving b.s. to the charge d'affaires but also to Iran.

I believe I speak for all Americans when I say the last thing this country wanted to do was send Ali Agah away mad.