What was originally added to rum to make grog?

Grog is any of a variety of alcoholic beverages. The word originally referred to a drink made with water and rum, which British Vice Admiral Edward Vernon introduced into the naval squadron he commanded in the West Indies on 21 August 1740. Vernon wore a coat of grogram cloth and was nicknamed Old Grogram or Old Grog. Merriam Webster's Collegiate Dictionary, which agrees with this story of the word's origin, states that the word "grog" was first used in this sense in 1770, though other sources cite 1749.

Sailors require significant quantities of fresh water on extended voyages. Since desalinating sea water was not practical, 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.

Following England'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 saved the rum rations for several days to drink all at once. Due to the subsequent illness and disciplinary problems, the rum was mixed with water. This both diluted its effects and accelerated its spoilage, preventing hoarding of the allowance.