congruent (adj.)

early 15c., "suitable, proper, harmoniously joined or related," from Latin congruentem (nominative congruens) "agreeing, fit, suitable," present participle of congruere "agree, correspond with," literally "to come together," from assimilated form of com "with, together" (see con-) + a lost verb *gruere, *ruere "fall, rush," which de Vaan traces to a PIE *ghr(e)uho- "to rush in." 

Geometry sense, "capable of being superposed," is attested by 1706. Related: Congruently.

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