The [Online Etymology Dictionary][1] suggests:

> **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 to a *conch* (pronounced "conk") shell.


  [1]: http://www.etymonline.com/index.php?allowed_in_frame=0&search=conk&searchmode=none