Ir*ra"tion*al (?), a. [L. irrationalis: cf. F. irrationnel. See In- not, and Rational.]
Not rational; void of reason or understanding; as, brutes are irrational animals.
Not according to reason; absurd; foolish.
It seemed utterly irrational any longer to maintain it.
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.
© Webster 1913.