n. Transposing sounds or letters in a word or phrase.
It is given to few men to donate their name to the language and most of them are rather proud to do so. Pity then William Archibald Spooner, warden of New College, Oxford, 1903-1924, whose gift to the world, as we were reminded last Monday by Jim Naughtie's slip, was not some mighty academic achievement, but the spoonerism.

The accidental transposition of part of the sounds of two words is technically metaphasis, but for more than 100 years it has been hung around the neck of an otherwise obscure classics don.
—“In praise of… Dr Spooner,” The Guardian, December 13, 2010
1986 (earliest)
SIR: Patrick Hughes is right (LRB, 24 July): ‘metaphasis’ is not in the OED. In fact, as far as I can see, it is not in any dictionary. But couldn’t it be that there is a distinction to be made between ‘metaphasis’ and ‘metathesis’? The OED defines the latter as ‘the interchange of position between sounds or letters in a word’ (my italics). An example would be Old English bridd becoming modern bird. This leaves ‘metaphasis’ free to describe what Spooner did: transpose sounds between different words, like his classic ‘our queer Dean’.
—Adrian Room, “Letters,” London Review of Books, October 23, 1986
Such phrases are known more popularly as "Spoonerisms" after the Reverend William Spooner who was famous for mangling phrases ("Which of us has not felt in his heart a half-warmed fish?").
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