hen's tooth

Example of a jocularly non-existent phenomenon, as many birds lack any teeth (having developed a bill instead). Compare horsefeathers and when pigs fly.

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hen's tooth (plural hen's teeth or hens' teeth)

1. (idiomatic) Anything not naturally occurring.

   Synonyms: rocking horse shit; see also Thesaurus:rarity

   as rare as a hen's tooth

   • 1992, Edwin Williamson, The Penguin history of Latin America, London, New York: Penguin Books, →ISBN, page 183:

     Indeed, many of the Dutch in Pernambuco were won over to the Church of Rome, ‘whereas converts from Catholicism to Calvinism were as rare as hens’ teeth’.

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