late 14c., "not likely to occur," from un- (1) "not" + likely (adj.). Similar formation in Old Norse ulikligr, Middle Danish uligelig. Meaning "not likely to be true" is recorded from 1590s. Related: Unlikeliness; unlikelihood.
mid-15c., "improbably," from un- (1) "not" + likely (adv.) (see likely (adj.)).
prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").
The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.
It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.
c. 1300, "having the appearance of truth or fact," perhaps from Old Norse likligr "likely," from likr "like" (see like (adj.)). Old English had cognate geliclic. Meaning "having the appearance of being strong and capable" is from mid-15c., though now mostly confined to American English; according to OED this sense is perhaps influenced by like (v.). Sense of "good-looking" ("such as may be liked") is from late 15c. Meaning "probable" is attested from late 14c., but said by OED to be now principally in American English. As an adverb, late 14c., from the adjective.
LIKELY. That may be liked; that may please; handsome. In the United States, as a colloquial term, respectable; worthy of esteem; sensible.—Worcester. [Bartlett]
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Harper, D. (n.d.). Etymology of unlikely. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/unlikely