A wheel with teeth that fits into other toothed wheels to transfer motion in a machine; also means a person who is just a small part of a big system.
From Old Norse 'koggr' originally meaning a wooden peg or tooth, evolved into the mechanical gear tooth, possibly from the sound or appearance.
The metaphor 'just a cog in the machine' became powerful in the Industrial Age because factory workers literally were cogs—replaceable, repetitive parts—which sparked entire philosophical movements about individuality and human worth.
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