The word "monkey" is of uncertain origin; its first known usage was in 1498 when it was used in the literary work Reynard the Fox as the name of the son of Martin the Ape. "Monkey" has numerous nautical meanings, such as a small coastal trading vessel, single masted with a square sail of the 16th and 17th centuries; a small wooden cask in which grog was carried after issue from a grog-tub to the seamen's messes in the Royal Navy; a type of marine steam reciprocating engine where two engines were used together in tandem on the same propeller shaft; and a sailor whose job involved climbing and moving swiftly (usage dating to 1858). A "monkey boat" was a narrow vessel used on canals (usage dating to 1858); a "monkey gaff" is a small gaff on large merchant vessels; a "monkey jacket" is a close fitting jacket worn by sailors; "monkey spars" are small masts and yards on vessels used for the "instruction and exercise of boys;" and a "monkey pump" is a straw used to suck the liquid from a small hole in a cask; a "monkey block" was used in the rigging of sailing ships; "monkey island" is a ship's upper bridge; "monkey drill" was calisthenics by naval personnel (usage dating to 1895); and "monkey march" is close order march by US Marine Corps personnel (usage dating to 1952).
[Sources: Cassidy, Frederick G. and Joan Houston Hall eds. Dictionary of American Regional English. vol.3 (Cambridge MA: Harvard University Press, 1996): 642; Wilfred Granville. A Dictionary of Sailors' Slang (London: Andre Deutch, 1962): 77; Peter Kemp ed. Oxford Companion to Ships & the Sea. (New York: Oxford University; Press, 1976): 556; The Oxford English Dictionary. New York: Oxford University Press, 1933; J.E. Lighter ed. Random House Historical Dictionary of American Slang. (New York: Random House, 1994): 580.; and Eric Partridge A Dictionary of Slang and Unconventional English. 8th ed. (New York: Macmillan Publishing Company): 917.]
"Monkey" has also been used within an ordnance context. A "monkey" was a kind of gun or cannon (usage dating to 1650). "Monkey tail" was a short hand spike, a lever for aiming a carronade [short-sight iron cannon]. A "powder monkey" was a boy who carried gun powder from the magazine to cannons and performed other ordnance duties on a warship (usage dating to 1682).
[Source: The Oxford English Dictionary. New York: Oxford University Press, 1933.]
The first recorded use of the term "brass monkey" appears to dates to 1857 when it was used in an apparently vulgar context by C.A. Abbey in his book Before the Mast, where on page 108 it says "It would freeze the tail off a brass monkey."
[Source: Lighter, J.E. ed. Random House Historical Dictionary of American Slang. (New York: Random House, 1994): 262.]
It has often been claimed that the "brass monkey" was a holder or storage rack in which cannon balls (or shot) were stacked on a ship. Supposedly when the "monkey" with its stack of cannon ball became cold, the contraction of iron cannon balls led to the balls falling through or off of the "monkey." This explanation appears to be a legend of the sea without historical justification. In actuality, ready service shot was kept on the gun or spar decks in shot racks (also known as shot garlands in the Royal Navy) which consisted of longitudinal wooden planks with holes bored into them, into which round shot (cannon balls) were inserted for ready use by the gun crew. These shot racks or garlands are discussed in: Longridge, C. Nepean. The Anatomy of Nelson's Ships. (Annapolis MD: Naval Institute Press, 1981): 64. A top view of shot garlands on the upper deck of a ship-of-the-line is depicted in The Visual Dictionary of Ships and Sailing. New York: Dorling Kindersley, 1991): 17.
"Brass monkey" is also the nickname for the Cunard Line's house flag which depicts a gold lion rampant on a red field.
[Source: Rogers, John. Origins of Sea Terms. (Mystic CT: Mystic Seaport Museum, 1984): 23.]