1520s, "to heap up" (transitive), from Latin accumulatus, past participle of accumulare "to heap up, amass," from ad "to," here perhaps emphatic (see ad-), + cumulare "heap up," from cumulus "heap" (from suffixed form of PIE root *keue- "to swell"). From 1759 in intransitive sense of "grow in size or number." Related: Accumulated; accumulating.
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).
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Harper, D. (n.d.). Etymology of accumulative. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/accumulative