When you are about to masturbate but you see something so horrendous that you stop.
“Hey dude, what happened, I thought you said you were gonna go masturbate?”
“I was, but I saw some weird foot stuff and stopped.”
“Oh! So you started Losing your Fappetite.”
To have the urge or desite to masturbate. Originates from the words fap (to masturbate) and appetite (the desire to eat food).
After Jim saw that pornstar shit on that dude's dick on PornHub, he knew he had lost his fappetite.
Oh man, Joe hasn't been laid in 6 weeks. He's got a huge fappetite.