Bonk the verb first came into the world – in print, at least – in 1929 as a ‘conversion’ of the British interjection of the same year, which is clearly onomatopoeic. As the OED comprehensively notes ‘Representing an abrupt, typically hollow-sounding, heavy thumping noise, as of a blow, or one hard or unyielding object striking another.’ The verb in this sense is ‘to strike something hard or unyielding’ and the second OED citation shows a writer trying to convey a sequence of disparate noises: The carrier men…bonked and rattled and squerked the package through the almost too small doorway and set it down with a thump. N. Hunter, Professor Branestawm's Treasure Hunt i. 13, 1937   The sexual meaning is first recorded from 1975. Its achievement, at least in Britain, was to give people a word they could use without stammering or blushing to describe an act which theretofore could only be described by euphemism, coarse slang or the starchy language of medicine. Here instead was a ‘fun’ word: short, somewhat childish yet sooo satisfying to say. It had a sort of ‘naughty but nice’ feel, risqué, but then again, not really. And it seemed a lot less crude and slangy than s**g. Dot Wordsworth waxes lyrical about it while surprised how many of the surveyed respondents appeared not to know it (37%).