What Word?

Today as I was taking a weekly walk with a friend around a track, which happens to have a park, my son was approached by bigger boys. These bigger boys, maybe about 10, used the "f" word. Yep, they dropped the "f" bomb. Now I know I can not protect my child from everything and that things are going to happen and that things are going to be said/heard, but what flabbergasted me the most was how my son told me of the occurrence.

Obviously, he told me the boys were saying bad words. Usually that means dumb, stupid, or even shut-up. So when I asked him what they said, his response was the "f" word. Now, I am thinking what in the world does my 6 year old know about the "f" word. Before I could even finish that thought he said it, "f_ _ _ing". Oh crap, tell me I did not hear what I thought I heard. But once again, before I could finish that thought he said it again, "f_ _ _ ing". How in the world did he know I needed some clarification? He did let me know they called him dumb, but my mind was still playing back his little voice saying "f_ _ _ing".