unlikely (adj.)

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.

unlikely (adv.)

mid-15c., "improbably," from un- (1) "not" + likely (adv.) (see likely (adj.)).

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Definitions of unlikely from WordNet

unlikely (adj.)
not likely to be true or to occur or to have occurred;
legislation on the question is highly unlikely
Synonyms: improbable
unlikely (adj.)
has little chance of being the case or coming about;
an unlikely candidate for reelection
an unlikely story
a butcher is unlikely to preach vegetarianism
unlikely (adj.)
having a probability too low to inspire belief;
Synonyms: improbable / unbelievable / unconvincing