Forming or moving in a group or gang; or working together as a coordinated group.
From the noun 'gang,' which comes from Old Norse 'gangr' meaning a going or journey. The sense of a 'group of people' developed from people who went places together, eventually taking on associations with criminal groups.
Gang originally just meant 'a group walking together,' but modern usage has so thoroughly criminalized it that we forget its neutral roots. It's a perfect example of how a word's meaning can shift based on which groups get associated with it historically.
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