Adjective
Archaic form of irrational.
(of an action or decision) Not rational.
1906, Report of the Minister of Agriculture for the Dominion of Canada, for the five months ended March 31, 1906, Department of Agriculture of Canada, page xxxvi:[…] there may be considerable loss of nitrogen due to removal of crops and irrationable methods of farming, […]
(of a person or thing) Not possessing rationality.
1860, Mann, Robert James, A guide to the knowledge of life, vegetable and animal, revised and corrected edition, page 344:Operations that are perfected in the economies of the irrationable animals, […]