fap · adjective1
Of unknown origin. First attested in 1616 in The Merry Wives of Windsor by William Shakespeare.
fap · interjection, verb, noun2
Onomatopoeic Internet neologism from the sound of penile 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. Compare Japanese しこしこ (shikoshiko).
Etymology from Wiktionary, CC BY-SA 4.0.