Fap is a slang term for male masturbation. The term alludes to the sound a man’s testicles make slapping against the body or against the hand when he pleasures himself. The earliest known use of the word fap appeared in the Japanese manga webcomic Sexy Losers in April 1999. The term has since spread to common mainstream use, particularly online.
Fap is the male equivalent of the similarly onomatopoeic terms for female masturbation, schlick, paff, and squish.