Loose-fitting pants, typically casual or business casual, made of soft fabric like cotton.
From 'slack' meaning 'loose' or 'not tight' (Old English 'slac'). The word shifted from an adjective describing loose fabric to a noun for the specific type of loose-fitting trousers.
Slacks got their name from the loose, comfortable fit—it's ironic that the word for 'loose' clothing became a formal enough word that you could wear them to an office, showing how clothing changes faster than the words for it!
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