Hit someone on the head, or (of an engine or machine) suddenly stopped working, usually unexpectedly.
From 'conk', possibly imitative of the sound of a blow to the head, or from nautical 'conk' meaning to strike. The 'breaking down' meaning comes from 20th-century slang.
The verb has two wild opposite meanings—a violent hit versus mechanical failure—and both became popular slang almost simultaneously in the early 1900s, showing how language loves unexpected word journeys.
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