Back when I was a little kid living in Queens, my parents were pretty good about keeping their mouths clean around me (my mom said "S-H-I-T" all the time, and "sugar!"), and I didn't get to watch any grown-up type movies. So when I was in first grade (or kindergarten, but it was probably first grade), I hadn't yet heard the F-word, I didn't know it existed.
One time I was in the bathroom at school, and there were a couple of Big Kids in there. One of them pointed to the side of a bathroom stall on which was scrawled "FUCK" and said, "Oooooooooooh, that's a bad word!"
I looked at it and read it, and said, "no it's not."
"Yes it is," she insisted. "It says, F-U-C-K."
"So? Fuck. That's not a bad word." All the girls' eyes got THIS big and they all pointed at me and made that little finger symbol that means "Oooooooh!" and said "Oooooooh! You said it! You said a very bad word! I'm telling."
Also in first grade I got in trouble for calling a fellow student a "chicken shit" during recess. I didn't fully realize the bad-ness of that bad word either, though I knew it was kinda bad. But I got into more trouble for that than I had anticipated.