cogwheel

Cogwheel is simply cog plus wheel. Cog traces back through Middle English cogge, Old Norse, and Proto-Germanic to a Proto-Indo-European root; wheel traces through Old English hwēol and Proto-Germanic back to the reduplicated Proto-Indo-European root for wheel, *kʷekʷléh₂.

The two words traveled separate paths for millennia — one for the tooth-like projection, one for the round rolling thing — before English fused them into a single word naming a toothed wheel that meshes with others.

Wheel shares its Proto-Germanic ancestor *hweulō with Old English hwēol, showing how deeply rooted the 'wheel' half of this compound is in English itself.