irrational (adj.) late 15c., “not endowed with reason” (of beats, etc.); earlier (of quantities) “inexpressible in ordinary numbers” (late 14c.); from Latin irrationalis “without reason,” from assimilated form of in- “not, opposite of” (see in- (1)) + rationalis “reason” (see rational). Meaning “illogical, absurd” is attested from 1640s. via etymonline.com… Continue reading