1530s, a back-formation from imitation or imitator, or else from Latin imitatus, past participle of imitari "to copy, portray" (from PIE root *aim- "to copy"). Related: Imitated; imitating. An Old English word for this was æfterhyrigan.
word-forming element making adjectives from verbs, meaning "pertaining to, tending to; doing, serving to do," in some cases from Old French -if, but usually directly from Latin adjectival suffix -ivus (source also of Italian and Spanish -ivo). In some words borrowed from French at an early date it has been reduced to -y (as in hasty, tardy).