1650s, "repetition of the same sound or letter at the beginning of words in close succession," from Modern Latin alliterationem (nominative alliteratio), noun of action from past-participle stem of alliterare "to begin with the same letter," from Latin ad "to" (see ad-) + littera (also litera) "letter, script" (see letter (n.1)). Related: Alliterational.
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/alliterative">Etymology of alliterative by etymonline</a>
Harper, D. (n.d.). Etymology of alliterative. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/alliterative