The fun thing about slang is the shades of meaning that certain words can take. "Cock" and "weewee" both mean penis, but you wouldn't use them interchangably. And if you would, tell me now, so I'd never go to bed with you.
I like the word "fap", because to me, it has shades of meaning that other terms for masturbation don't. To me, it doesn't just mean "to masturbate". I've always taken it to mean "to masturbate somewhat uncontrollably, in response to an unexpected stimulus". So you might be looking at (non-sexual) stuff online, and suddenly you come across a picture that makes you so horny, you almost unconsciously throw your pants down, grab your dick, and start stroking it so fast that your balls start slapping. (Making that "fap fap fap" sound from whence the term came.) And post-orgasm, you sit there and wonder "What the hell came over me?"
It's funnier if used in context to something you wouldn't expect to find horny at all. "I can't believe I just fapped to C-SPAN."
Lex