Defined by the Oxford English Dictionary, second edition, 1989:
wank, n. (a.) Also whank.
1. Of a male: (an act of) masturbation. This word and its derivatives are not in polite use. 1951 Dict. Slang (ed. 4) 1220/1 Whank, (male) self-abuse: low: from ca. 1870. Perhaps echoic.
I was going home to whank, but was distracted by the house that was on fire, and the escaping giraffes.