What is F-teen?
The age at which a young lady(a teenager) is considered a 'viable' conquest by a grown man.
"Hey, Adam, how old is that younger sister of yours?"
"She's not f-teen yet mo-fo, so stay the fuck away from her."
"Whoa, whoa, whoa... you thought that girl with her learners permit was f-teen. Isn't your sister almost out of high school?"
"Yeah... go ahead and mac down on her..."