My oldest's first was "shit".
"Do you know what that means?"
"Yeah! You say it when you drop something, or you're scared."
"Fair enough."

Haha yes, I drop curse words rather often and he's picked up on a few over the years. The rule now is that he can use curse words if they're not targeted AT a person and it's used appropriately ie saying "Damnit!" when he stubs his toe.