improbability (n.)youdaoicibaDictYouDict[improbability 词源字典]
1590s, "fact or quality of being improbably;" see improbable + -ity. Meaning "an instance of something improbable" is from 1610s.[improbability etymology, improbability origin, 英语词源]
improbable (adj.)youdaoicibaDictYouDict
1590s, from assimilated form of in- (1) "not, opposite of" + probable, or else from Latin improbabilis. Related: Improbably.
unlikely (adv.)youdaoicibaDictYouDict
mid-15c., "improbably," from un- (1) "not" + likely (adv.) (see likely (adj.)).