Hitting hard and forcefully, especially with the fist; also means moving slowly or heavily.
Origin uncertain but possibly from Scandinavian languages; related to the noun 'slug' meaning a hard blow. The modern sense of hitting may derive from 'slug' as a term for a heavy blow or impact.
In baseball slang, a slugger is a power hitter, but the word has humble origins—nobody's quite sure if it comes from actual slugs (which are soft and slow) or from Scandinavian fighter terminology. It's one of those delightfully murky words where etymology gets charmingly lost.
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