Past tense of slug: hit hard with a fist or blunt object, or moved slowly and laboriously.
From 'slug' meaning either a hard blow (possibly from slug the mollusk, suggesting something heavy) or slow movement (from the slow-moving gastropod). Both senses developed in American English in the 19th century.
The dual meaning of 'slug' - both hitting hard and moving slowly - seems contradictory but both come from the image of something heavy and solid. Baseball's 'slugger' combines both concepts: powerful, deliberate hitting.
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