1620s, "quality of having reason;" 1650s, "fact of being agreeable to reason," from French rationalité, from Late Latin rationalitas "reasonableness, rationality" (also source of Spanish racionalidad, Italian razionalita), from Latin rationalis (see rational).
his rationality may have been impaired
ration
rational
rationale
rationalise
rationalist
rationality
rationalization
rationalize
rationing
ratline
rat-pack