1767, "explain in a rational way, make conformable to reason," from rational + -ize. The psychological sense of "to give an explanation that conceals true motives" dates from 1922, on the notion of "cause to appear reasonable or socially acceptable." In 19c. Newman and Browning also used it with an intransitive sense of "think for oneself, employ one's reason as the supreme test." Related: Rationalized; rationalizer; rationalizing.