Conk

Definition

The origin of 'conk' is uncertain, although one theory is that conk is derived from Conkers or horse chestnuts, which are used in a child's game in which one person uses his/her conker to smash those of opposing players. Also, "nut" means head, as in, "watch your nut" (watch your head), "He's off his nut" (he's crazy). (contributed by Pat Gilliland) To "conk out," means to pass out.