"opening, grated gate, half-door," Old English hæc (genitive hæcce) "fence, grating, gate," from Proto-Germanic *hak- (source also of Middle High German heck, Dutch hek "fence, gate"), a word of uncertain origin. This apparently is the source of many of the Hatcher surnames; "one who lives near a gate." Sense of "opening in a ship's deck" is first recorded mid-13c. Drinking phrase down the hatch attested by 1931 (the image is nautical).