congruous 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.

word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."