Navy grog is a tiki cocktail built on three or more rums, citrus juice, and a spiced sweetener, created in the early 1940s by Don the Beachcomber and later popularized in a drier, spicier form by Trader Vic's. Despite the name, it is a distinct drink from the historical Royal Navy grog it pays tribute to, not a recipe for that ration itself.
The original grog was a much simpler mixture: rum diluted with water, issued as a daily ration to British Royal Navy sailors from 1740 until 1970. The ration is named for Admiral Edward Vernon, nicknamed "Old Grog" for the grogram cloak he wore, who ordered the rum diluted to curb drunkenness aboard ship. Lime juice and sugar were later added to the ration, both to mask the taste of brackish water and to help prevent scurvy.
The tiki cocktail that borrows the name is considerably more elaborate. Don the Beachcomber's version mixed three types of rum with lime and grapefruit juices and honey; Trader Vic's later take swapped the honey for allspice dram, giving his rendition a drier, spicier profile. Both are served chilled over crushed ice and bear little resemblance, beyond containing rum and citrus, to the plain watered-down ration sailors once received.
Today navy grog remains a staple of tiki bars and specialty rum houses, valued for its balance of citrus, spice, and multiple rums. It is best understood as a 20th-century tiki invention that references naval history rather than as a historical naval recipe in its own right.