E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
in sailor language, is the vessel which contains the full allowance of grog. Halliwell (Archaic Dictionary) has
Moncorn, Beere corne, barley bygge, or moncorne.(1552.)
To suck the monkey. Sailors call the vessel which contains their full allowance of grog a monkey. Hence, to suck the monkey is surreptitiously to suck liquor from a cask through a straw. Again, when the milk has been taken from a cocoanut, and rum has been substituted, sucking the monkey means drinking this rum. Probably monkey in all such cases is a corruption of moncorn (ale or beer). (See Marryats Peter Simple.) (See MONKEY SPOONS.)