conk (v.)

as in conk out, 1918, coined by World War I airmen, perhaps in imitation of the sound of a stalling motor, reinforced by conk (v.) "hit on the head," originally "punch in the nose" (1821), from conk (n.), slang for "nose" (1812), perhaps from fancied resemblance of the nose to a conch (pronounced "conk") shell. Perhaps also imitative: Compare Greek konk, a syllable representing the sound made by a pebble striking the bottom of the (metal) voting urn [William Smith, "Dictionary of Greek and Roman Antiquities"].