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.
late 14c., "formal or solemn pledge, promise," also "certainty, full confidence," from Old French asseurance "assurance, promise; truce; certainty, safety, security" (11c., Modern French assurance), from asseurer "to reassure, to render sure" (see assure). The meaning "self-confident" is from 1590s. The word had a negative tinge 18c., often suggesting impudence or presumption.
<a href="https://www.etymonline.com/word/self-assurance">Etymology of self-assurance by etymonline</a>
Harper, D. (n.d.). Etymology of self-assurance. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/self-assurance