Revised May 27 2021
"Sailors require significant quantities of fresh water on extended voyages. Since distilling sea water was too slow and fuel intensive, fresh water was taken on board in casks but quickly developed algae and became slimy. Stagnant water was sweetened with beer or wine to make it palatable which involved more casks and was subject to spoilage. As longer voyages became more common, the task of stowage became more and more difficult and the sailors' then-daily ration of a gallon of beer began to add up.
"Following Britain's conquest of Jamaica in 1655, a half pint or "2 gills" of rum gradually replaced beer and brandy as the drink of choice. Given to the sailor straight, this caused additional problems, as some sailors would save up the rum rations for several days, then drink them all at once. Due to the subsequent illness and disciplinary problems, the rum was mixed with water. This both diluted its effects, and delayed its spoilage. A half pint (current American measurement; the larger British "Imperial" pint was not introduced until 1824), of rum mixed with one quart of water and issued in two servings, before noon and after the end of the working day, became part of the official regulations of the Royal Navy in 1756 and lasted for more than two centuries. This gives a ratio of 4:1 (water:rum).
"Citrus juice (usually lime or lemon juice) was added to the recipe to cut down on the water's foulness. Although they did not know the reason at the time, Admiral Edward Vernon's sailors were healthier than the rest of the navy, due to the daily doses of vitamin C that prevented disease (mainly scurvy). This custom, in time, got the British the nickname limeys for the limes they consumed.
"The name "grog" probably came from the nickname of Admiral Vernon, who was known as 'Old Grog' because he wore a grogram cloak. American Dialect Society member Stephen Goranson has shown that the term was in use by 1749, when Vernon was still alive. A biographer of Daniel Defoe has suggested that the derivation from 'Old Grog' is wrong because Defoe used the term in 1718, but this is based on a miscitation of Defoe's work, which actually used the word 'ginger.'"