"Watch your mouth," is what we used to say as kids. It was recognition of the truth that words in a street-corner, playground society could box you into a corner, leaving you no choice under the rules of machismo or even, if you will, boychismo, but to fight. After all, to lose a fight is painful; to back down, though, is even more painful.

Somewhat the same rules apply to nations. And it applies in spades to the United States which, like the king of the playground, can least afford not to back its words with action. The eyes of the world are upon us.

And the ears of the world have been hearing an escalation of rhetoric about Central America that could, if it continues, leave us no option but to do what President Reagan says he has no intention of doing--send in the troops. He reiterated that pledge at his most recent press conference, but then, two days later talked as if he might, after all, have no choice.

"U.S. security--the safety of American citizens--that's why Central America matters so much," Reagan said in a speech in Long Beach, Calif. "Either we pay a modest price now so we can prevent a crisis or we listen to the do-nothings and risk an explosion of violence that will bring real danger to our own borders." As if that was not clear enough, the president added, "We must not turn our backs on our friends."

For a president who at his news conference said that a president should never say never, all this has the ring of finality to it. Never mind the president's arguments and never mind the questions many experts have about the basis for his fear, the fact remains that in words alone the president has assumed an Alamo stance: We should never "turn our backs on our friends" in Central America.

The trouble with this, as Piero Gleijeses pointed out in an article in Foreign Affairs magazine, is that it contradicts the president's pledge not to send American troops to Central America. Gleijeses, an expert on the area, said the president cannot at one and the same time promote an area's importance so that its loss represents "a risk to our own borders" and, at the same time, say we will not send troops to keep it secure.

There is a gap in logic here. It is, in fact, the sort of gap that led us directly into the Vietnam quagmire when that country, too, was promoted early on as being absolutely essential to the security of the United States (we had to stop the commies before they reached San Francisco), but not so essential that large numbers of American troops had to fight there. After a while, though, the gap was closed--by sending in the troops.

That may not happen in Central America. But the fact of the matter is that for all our fondest wishes the Sandinista regime in Nicaragua still enjoys popular support and the friendly regime in El Salvador rests on a bedrock of indifference. Neither American military aid nor economic aid has been able to break the stalemate with the guerrillas in El Salvador and Guatemala has really yet to be heard from. In short, things are simply not going our way.

At the moment, the administration is trying to get Congress to see El Salvador as it does. It wants $110 million more in military aid and it is worried that, lacking popular support for its programs, Congress will be under no compunction to go along. So the president is taking his case to the people, trying to build popular support for his programs--trying, in other words, to turn the heat on Congress.

The problem is that both at home and abroad he has so much hyped the importance of Central America that there might be no way to turn back. How, with all the world watching, can we raise the importance of Central America to the level of crucial and then stop short of sending in troops? How can we pledge not to "turn our backs on our friends" and then turn our backs on them when what they might need is American soldiers?

The answer is that the president's words may compel him to do what he says he does not want to do. The world is no playground, but words can still paint you into a corner. Even for a president, it is never too late to watch your mouth.