Irrational, a.
Etymology: [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.
1 more senses
Syn. -- Absurd; foolish; preposterous; unreasonable; senseless. See Absurd.
Shades of meaning
Fernald has no entry of its own for irrational; he lists it under absurd.
Where he writes about it:
Anything is irrational when clearly contrary to sound reason, foolish when contrary to practical good sense, silly when petty and contemptible in its folly, erroneous when containing error that vitiates the result, unreasonable when there seems a perverse bias or an intent to go wrong. — from absurd