When DS was 2 he went around saying "F" a lot. A whole lot. The more I'd tell him to stop or react at all the more he said it. one day in particular he wouldn't stop, walked around just saying "f" "f" "f" "f"... I said WHY are you saying that, STOP! He said "but mommy I just LOVE that word".
where did he get it? Yep, either DH or me or who knows. I'll admit, I'm not a perfect parent and sometimes I slip up. Am I proud? nope but i'm not going to claim the OP is a horrible person while i'm angelic!
Anyway I told him it is not a nice word and even grown ups shouldn't say it. Then we decided to ignore him. a few days later he stopped and has not said it since, he is now 7.
Sure occassionally he has let a word slip here or there, now he is into spelling them. We just tell him "that is not a word for you" and we all move on.
Now just so everyone doesn't think I cuss like a sailor, my DD9 did not go through any of that!