unsparing (adj.)youdaoicibaDictYouDict[unsparing 词源字典]
"showing no mercy," 1580s, from un- (1) "not" + sparing, attested from late 14c. as a present participle adjective from spare (v.). Meaning "profuse" is from 1660s. Related: Unsparingly.[unsparing etymology, unsparing origin, 英语词源]