ruthless (adj.)youdaoicibaDictYouDict[ruthless 词源字典]
early 14c., from reuthe "pity, compassion" (see ruth) + -less. Ruthful (early 13c.) has fallen from use since late 17c. except as a deliberate archaism. Related: Ruthlessly; ruthlessness.[ruthless etymology, ruthless origin, 英语词源]