rhotacism (etymology)
Definition - The tendency in the evolution of spoken language for the /r/ sound to take the place of other sounds, especially the /s/ sound.

Example -
The Old English wære — which became were — derives from the Old English verb wesan, to be.

Etymology -
The word entered English via Modern Latin from the Greek rhotakizein (from rho, the Greek name for the letter R).

Oxford English Dictionary -
Its first citation in this sense is from 1844:
"The same tendency to rhotacism, which is characteristic of the Umbrian, Doric and Old Norse dialects."
(Donaldson Varronianus 205)

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