A snack that is eaten while in the middle of a vigorous masterbating session. it is usually something small and able to be eaten one handed, sandwiches are a good Fap Snack.
Guy 1: Dude, last night , I was fapping so hard that I needed to eat me a sandwich while I did it.
Guy 2: Ah ok. So you needed to have yourself a Fap Snack.