1728, "to make better," in some cases perhaps a back-formation from amelioration on pattern of French améliorer, or else from Medieval Latin amelioratus, past participle of ameliorare. The intransitive sense of "grow better" is by 1789. The simpler form meliorate was used in Middle English. Related: Ameliorated; ameliorating; ameliorable.
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).
<a href="https://www.etymonline.com/word/ameliorative">Etymology of ameliorative by etymonline</a>
Harper, D. (n.d.). Etymology of ameliorative. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/ameliorative