From Longman Dictionary of Contemporary Englishmercilessmer‧ci‧less /ˈmɜːsɪləs $ ˈmɜːr-/ adjective 1 CRUELcruel and showing no kindness or forgiveness a merciless attack a merciless killer2 → merciless heat/cold/wind etc —mercilessly adverb He teased his sister mercilessly.
Examples from the Corpus
merciless• He was very sweet; sweet and merciless.• They gave Donlan the same merciless beating Clark received in Chicago.• And again events have dealt them a series of merciless blows.• He had a cruel, clever, merciless face, with a big curved nose and very bright, hard eyes.• I have suffered as a result of this merciless hard line plenty of times myself.• The furies had claws and merciless teeth. 