To hit someone on the head, or for an engine to stop working suddenly (as in 'conk out').
Possibly from 'conch' (the shell) used metaphorically for hitting on the head like a shell. May also relate to Afrikaans 'konk.' The origin is somewhat uncertain but likely imitative.
'Conk out' became really popular during World War I when engines would fail—soldiers described their engines stopping with this onomatopoetic word that sounds almost like an engine failing!
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