Marked with a label or sticker, or in the game of tag, touched by the person who is 'it.'
From Middle English 'taggen,' possibly related to Old Norse 'taggr' meaning 'point' or 'spike.' The modern sense comes from the idea of attaching a marker to something.
The word 'tag' jumped from meaning a physical label to social media tagging to the children's game—it's one of those everyday words that accidentally became connected to how we organize and identify things in the modern world.
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