late 14c., from Old French suasion (14c.) and directly from Latin suasionem (nominative suasio) "a recommending, advocacy, support," noun of action from past participle stem of suadere "to urge, incite, promote, advise, persuade," literally "recommend as good" (related to suavis "sweet"), from PIE root *swād- "sweet, pleasant" (see sweet (adj.)). Survives chiefly in phrase moral suasion (1640s). Latin Suada was the goddess of persuasion.
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/suasive">Etymology of suasive by etymonline</a>
Harper, D. (n.d.). Etymology of suasive. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/suasive