According to Billy Connolly it's the noise the bedsprings make when the act is taking place....no seriously it seems to have entered the spoken language in the late 19th century, significantly at a time when the word Whang was emerging as a vulgar term for penis. W@nk (earlier spelled 'whank') is probably derived from the same source 'whang' as a dialect word first meaning hit, beat or slap. W@nk may be simply a variant pronunciation, or a development of the earlier word influenced by 'whack' and 'yank'.