Old variant of word wank, meaning
masturbate. Can be a noun or verb: a whank; to whank.
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.