c. 1600, "declare;" 1640s, "vindicate, maintain, or defend by words or measures," from Latin assertus, past participle of asserere/adserere "to claim, lay claim to, appropriate," from ad "to" (see ad-) + serere "to join together, put in a row" (from PIE root *ser- (2) "to line up"). Related: Asserted; asserting. To assert oneself "stand up for one's rights or authority" is recorded from 1879.
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).