Possibly an onomatopoeic imitation of the sound of a stalling internal combustion engine. Possibly coined by British motorcyclists circa 1910.
conk out (third-person singular simple present conks out, present participle conking out, simple past and past participle conked out)
- (intransitive, informal) to fall fast asleep; to sleep soundly
- He wasn't there, just the dog, conked out on the porch.
- (intransitive, informal) to stop functioning
- The old car conked out half way up the hill.
- (intransitive, informal) to die
- He lived to be 90 then conked out in his sleep.