1767, "explain in a rational way, make conformable to reason," from rational + -ize. In the psychological sense of "to give an explanation that conceals true motives" it dates from 1922. Related: Rationalized; rationalizing.
rationalize the child's seemingly crazy behavior
he rationalized his lack of success
We rationalized the factory's production and raised profits
When one wonders why one is doing certain things, one should rationalize
This function can be rationalized