I agree with Sandman, not enough info. Schools are like this from my experience and of course you were not there to know the WHOLE situation. You will never know all that happened or precipitated the event.
thank you so much for the answer, i was discombobulated, do not know whether i ll take a severe action and punishment or just talk to him about it and teach him it s wrong. well i did what you suggested , thank you again
Essentially, you don't have the information to make a decision. It would not be unusual for a child of his age to use a word he heard on the playground which all the kids reacted to, and not have a clue what it meant. Also if the teacher did not hear him say the word(s), there is no way to know if the report was accurate. I am surprised the school even notified you if the account was based on hearsay or second hand evidence.
Either way, you don't punish him. You do let him know that the word in question is a word he should never say and if you ever hear reports of him using that word again, he will be punished. Teacher wise - unless she brings it up again, I would ignore it. She was probably just doing her duty to keep parents informed.