Definition - The reordering of sounds within a word, especially as a process of language change.

Example -

(1) flimsy was probably coined as a metathesis of film
(2) crud from curd
(3) third from the Old English thridda
(4) horse from the Old English hros.

Etymology -
The word ultimately derives from the Greek metathesis, change of position (from meta, to change + tithenai, to place or set).
Note: The Greek term was coined by Dionysius of Halicarnassus, a first-century BCE Greek scholar who analyzed and modified texts to make them more eloquent. He called his editing methodology metathesis.

Oxford English Dictionary -
Its first citation in this sense is from 1660:
"Tahur, which is the Metathesis of Hurta, a thief."
(Jer. Taylor Duct. Dubit. iv. i. rule 2 §36)

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