What is an oversounding, or a misuse, of the letter r; specifically (Phylol.), the tendency, exhibited in the Indo-European languages, to change s to r, as wese to were called?

rhotacism — An oversounding, or a misuse, of the letter r; specifically (Phylol.), the tendency, exhibited in the Indo-European languages, to change s to r, as wese to were. (Webster's 1913.)

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