Ir‐ra″tion‐al (ĭr‐răsh″ŭn‐al), a. [L. irrationalis: cf. F. irrationnel. See In- not, and Rational.]
1. Not rational; void of reason or understanding; as, brutes are irrational animals.
2. Not according to reason; absurd; foolish.
It seemed utterly irrational any longer to maintain it. I. Taylor.
3. (Math.) Not capable of being exactly expressed by an integral number, or by a vulgar fraction; surd; — said especially of roots. See Surd.
Syn. — Absurd; foolish; preposterous; unreasonable; senseless. See Absurd.