unfathomable (adj.)youdaoicibaDictYouDict[unfathomable 词源字典]
1610s, originally in the figurative sense, of feelings, conditions, etc., from un- (1) "not" + fathomable. Literal sense attested from 1670s. Related: Unfathomably.[unfathomable etymology, unfathomable origin, 英语词源]