word forming element indicating "oneself," also "automatic," from Old English use of self (pron.) in compounds, such as selfbana "suicide," selflice "self-love, pride, vanity, egotism," selfwill "free will." Middle English had self-witte "one's own knowledge and intelligence" (early 15c.).
OED counts 13 such compounds in Old English. Middle English Compendium lists four, counting the self-will group as a whole. It re-emerges as a living word-forming element mid-16c., "probably to a great extent by imitation or reminiscence of Greek compounds in (auto-)," and formed a great many words in the pamphlet disputes of the 17c.
mid-15c., "significant, of much import, bearing weight or consequence," from Medieval Latin importantem (nominative importans) "important, momentous," present-participle adjective from importare "be significant in," from Latin importare "bring in, convey, bring in from abroad," from assimilated form of in- "into, in" (from PIE root *en "in") + portare "to carry," from PIE root *per- (2) "to lead, pass over." The meaning "pretentious, pompous" is from 1713. Related: Importantly. Compare import (v.) and (n.).
<a href="https://www.etymonline.com/word/self-important">Etymology of self-important by etymonline</a>
Harper, D. (n.d.). Etymology of self-important. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/self-important