Etymology 2

An echoic Internet slang and neologism from the sound of male masturbation, originally used in English translations of some adult Japanese manga, and popularized on the Internet by the webcomic The Thin H Line/Sexy Losers and other online sources.

e·cho·ic/eˈkō-ik/
Adjective:

Of or like an echo.
Imitating a sound; onomatopoeic.

Fapping got repeated a lot and it stuck like most common slang.