infallible (adj.)youdaoicibaDictgodict[infallible 词源字典]
early 15c., from Medieval Latin infallibilis, from in- "not, opposite of" (see in- (1)) + Late Latin fallibilis (see fallible). In reference to Popes, attested from 1870. Related: Infallibly.[infallible etymology, infallible origin, 英语词源]