c. 1400, "a bruising, contusion (to the head)," from Latin concussionem (nominative concussio) "a shaking, an earthquake," noun of action from past-participle stem of concutere "shake violently," from assimilated form of com "with, together" (see con-) + quatere "to shake" (see quash).
From late 15c. as "act of shaking or agitation," especially by impact of another body; from 1540s as "brain injury caused by a fall or blow."
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/concussive">Etymology of concussive by etymonline</a>
Harper, D. (n.d.). Etymology of concussive. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/concussive