unspeakable (adj.)youdaoicibaDictYouDict[unspeakable 词源字典]
c. 1400, "inexpressible," from un- (1) "not" + speakable (see speak (v.)). Meaning "indescribably bad or wicked" is recorded from mid-15c. Related: Unspeakably.[unspeakable etymology, unspeakable origin, 英语词源]