1
general::
Drink up, as in ‘‘Down the hatch," said Bill, as they raised their glasses. This phrase, often used as a toast, employs hatch in the sense of "a trap door found on ships." [Slang; c. 1930]
American Heritage Idioms
2
general::
Phrase(s): Down the hatch.
I am about to drink this.; Let’s all drink up. (Said as one is about to take a drink, especially of something bad-tasting or potent. Also used as a jocular toast.) • Bob said, "Down the hatch," and drank the whiskey in one gulp. • Let’s toast the bride and groom. Down the hatch!
McGrawhill's American Idioms And Phrasal Verbs