In a skillful and competent manner; doing something well and effectively.
From Middle English 'able' (from Old French 'able', from Latin 'habilis' meaning 'easily handled') plus the adverbial suffix '-ly'. The word has meant 'competently' since at least the 14th century.
The word 'able' originally meant 'easily handled' in Latin—imagine something that fits perfectly in your hand. Over centuries, this evolved into meaning 'capable' and eventually 'skilled,' showing how physical ease became mental competence in language.
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