E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Monkey,
in sailor language, is the vessel which contains the full allowance of grog. Halliwell (Archaic Dictionary) has
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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.)