"self-confident, feeling confident and assured as to oneself," 1711, from self- + assured.
Entries linking to self-assured
self-
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.
assured adj.
late 14c., of persons, "confident, self-assured," past-participle adjective from assure. From early 15c. as "secure, made safe." Related: Assuredly; assuredness.