1590s, from support (v.) + -ive. Called "rare" in OED 1st edition and Century Dictionary. Related: Supportively; supportiveness.
Entries linking to supportive
support v.
late 14c., "to aid," also "to hold up, prop up, put up with, tolerate," from Old French suporter "to bear, endure, sustain, support" (14c.), from Latin supportare "convey, carry, bring up, bring forward," from assimilated form of sub "up from under" (see sub-) + portare "to carry," from PIE root *per- (2) "to lead, pass over." Related: Supported; supporting.
-ive
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).