Putting a label, mark, or name on something; in modern usage, also means graffiti spray-painting, or labeling people in photos online.
From Middle English 'tag' possibly related to Old Norse 'tagg' (point or branch). The word originally meant a hanging piece of cloth or rope, then evolved to mean a label or mark attached to something.
The word 'tagging' has three completely different modern meanings—graffiti tagging, photo tagging, and retail price tagging—but they all come from the same core idea of attaching a label to claim ownership or identification!
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