The F Word

Last night, my seven-year old, out of the blue, mind you, opens up an interesting topic of discussion. Let me replay the incident for you within quotation marks:
AJ: Mom, how do you spell fuck?
(Emphasis was placed on the word DO, as if he had been contemplating this earth shattering question for days now in his young life.)
After my eyeballs were suctioned back into my head by the inhalation of my breath, I said, "Where did you ever hear that?"
AJ: (shrugging) I dunno.
I pressed further: "Come on, where did you hear that?"
AJ: Nowhere.
Me: AJ?
Unable to come up with a logical answer he lifts his shoulders and his eyebrows, almost as if a question and says: "You?"
Me: "You never heard me say that!"
AJ: "Yeah, remember the other day, you started to go 'ffffffffrrrr'?"
Me: "I was going to say FREAK."
Then I sat him down and said, "AJ, that is the baddest, baddest worst bad word in the entire world. Worse than stupid or dumb or hate or fart or crap and I never want you to say it again, okay?"
AJ: "Okay."
Me: "I'm not mad at you. I'm glad we had this talk."
Fuck, what am I going to do now?