In a way that shows no pity or compassion; without mercy or hesitation.
From 'ruthless' (lacking 'ruth,' an Old English word meaning 'pity' or 'compassion'), plus the adverb suffix '-ly.' 'Ruth' comes from Old English 'hreow.' The prefix 'ruth-less' literally means 'without pity.'
Most people don't know that 'ruth' is still a real word—it appears in literature meaning compassion or regret, but it's almost completely disappeared from modern English. Its opposite 'ruthless' survived while the positive form died out, which is weird because we usually keep both versions of words.
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