Slang term from historical thief's jargon meaning darkness or night, used as an opportunity for stealing.
From 'dark' + 'man' (an obsolete slang term) or possibly from Dutch, emerging in 16th-17th century thieves' cant and criminal argot before being recorded in dictionaries.
This word is pure crime jargon from centuries ago—thieves had entire secret languages with words like 'darkmans' for night and 'lightmans' for day, a linguistic underworld most people never knew existed!
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