lock

The lock that fastens a door comes from Old English loc, from Proto-West Germanic *lok, from Proto-Germanic *luką, from Proto-Indo-European *lewg- ("to bend; turn"). A second, separate lock — a lock of hair — comes from Old English locc, from Proto-West Germanic *lokk, from Proto-Germanic *lukkaz, tracing back to the same PIE root *lewg-.

The fastening sense stayed close to its root meaning of enclosing or shutting something in, while cognates elsewhere kept a related idea of a hole or opening — the two senses split from the same bending, turning notion. The hair sense, meanwhile, may connect to an old Gothic verb meaning "to shut," in the sense of curbing or containing hair.

Its Germanic cousins include German Loch and Dutch lok, both meaning "hole" — a reminder that closing something and the gap it closes were once the same idea.