Ir-rā'tion-al, a.
(-rāsh'un-al), a. [Prefix in, not, and rational; Lat. irrationalis, It. irrazionale, Sp. irrational, Pr. irrational, Fr. irrationnel.]
Sense 1. Not rational; void of reason or understanding; as, brutes are irrational animals.
Sense 2. Not according to the dictates of reason; contrary to reason; absurd.
The companions of the ministry of Jesus knew far too much of his divine power and majesty to throw up their profession of his Messiahship, even when it seemed utterly irrational any longer to maintain it. I. Taylor.
Sense 3. (Math.) Not capable of being exactly expressed by an integral number, or by a vulgar fraction; surd;—said especially of roots. See SURD.
Synonyms—Absurd; foolish; preposterous; unreasonable. See ABSURD.