“Who’da thunk it”, also written “who’d have thunk it” or “who’d a thunk it”, is a jokey, deliberately ungrammatical way of saying “who would have thought it?”. The mangled verb is the whole point: by swapping “thought” for the mock-illiterate “thunk”, the speaker signals that the surprise should not be taken too solemnly. It serves for genuine astonishment delivered with a wink, but at least as often it is pure sarcasm about something anyone could have predicted: “The budget airline cancelled the flight. Who’da thunk it?” It is informal everywhere English is spoken, and self-aware; nobody who says “thunk” believes it is a real past tense.
What’s the origin of the phrase ‘who’da thunk it’?
There was never a time when “thunk” was respectable English. The past tense of “think” has always been “thought”, and “thunk” is a deliberate mistake, coined for comic effect on the model of strong verbs such as drink/drunk and sink/sunk. The Online Etymology Dictionary describes it as a dialectal or jocular form made “by analogy of drink/drunk, sink/sunk”, and notes that it was recorded as a Yorkshire dialect form current in 1876, so the joke may have piggybacked on a scrap of genuine regional speech.
It was American humour, though, that made “thunk” famous, and it arrived already wearing this exact phrase. The earliest example in Green’s Dictionary of Slang is from the Lantern, a New Orleans newspaper, of 15 October 1887: “Who’d a thunk it!” Comic mangling of grammar was a staple of American newspaper humour in that era, and this specimen stuck.
By the 1910s the exclamation was a stock gag in the American popular press. The film trade paper Moving Picture World greeted a publicity photograph in 1912 with “Well, who’d a’ thunk it?”, and variants turn up across the decade in Motography, Motion Picture Magazine and Photoplay. The joke crossed the Atlantic and even reached high literature: in James Joyce’s Ulysses (1922), the hallucinated Virag barks at Bloom:
Stop twirling your thumbs and have a good old thunk.
Green’s Dictionary of Slang records the phrase’s later spellings settling down: “Who’d a thunked it?” in a James T. Farrell short story written in 1932, and the contracted “Who’da thunk it?” in the New York Age, a New York newspaper, by 1936.
The line is popularly remembered as a favourite of Mortimer Snerd, the amiably dim ventriloquist’s dummy Edgar Bergen introduced on American radio in the late 1930s. Snerd’s slack-jawed amazement certainly suits it, and he may well have carried it to a bigger audience, but no invention was involved: the phrase had been in print for half a century before he existed. By 1948 RCA Victor was listing a novelty record titled “Who’d a Thunk It”, sung by Helen Carroll and the Satisfiers, in its catalogue. The phrase has never needed reviving since; its ironic register, mock surprise at the entirely foreseeable, is if anything more common today than the sincere kind. When the surprise is genuine, the bookmakers long ago supplied English with an alternative: a turn-up for the books.