Crazy white people who perform hairbrained stunts to impress people, usually frat guys or dropouts. See Jackass
Glen: I don't know, what?
Truman: Dude! Check this shit out!
(2) To disprove a suspected prinny or flatspot.
"Oh, nothing really. We hung out for a while, I made sure she was satisfied, and we went to sleep."
"Oh man! You FWPed the shit out of her!!"
basically means that you're friends, but both of you know that there's a possibility of you becoming an item in the future.
julia: we decided on being fwp, so we'll see where it goes from there
Usual indicators are found in the ankles and cheeks in particular