congruent: [15] Etymologically, triangles that are congruent ‘come together’ or ‘agree’ – that is, are similar. The word comes from congruēns, the present participle of Latin congruere ‘come together, meet, agree’. This was a compound verb formed from the prefix com- ‘together’ and a verb, *gruere, not found elsewhere (some have linked it with Latin ruere ‘fall’ – ultimate source of English ruin – in which case congruere would have meant literally ‘fall together’, but others have seen a connection with Greek zakhrēēs ‘attacking violently’). Incongruous is a 17thcentury adoption from Latin incongruus.
congruent (adj.)
early 15c., "suitable, proper," from Latin congruentem (nominative congruens) "agreeing, fit, suitable," present participle of congruere, literally "to come together, agree, correspond with," from com- "with" (see com-) + a lost verb *gruere, *ruere "fall, rush," perhaps from PIE *ghrei- "to rub, grind" (see chrism). Geometry sense attested by 1706.
