In 1655, the Royal Navy began issuing a half-pint of rum to its sailors each day, to replace the beer or wine that had previously been issued. Originally, the alcohol was carried to hide the flavor of water that had gone stagnant on a long voyage, but the practice continued well after that issue had been dealt with.
Issues arose from sailors saving up their rum rations to drink several days worth at once, so the rum was cut with water, in a roughly 4:1 ratio, or roughly 20% rum. The drink became known as grog in the mid-1700s, probably in reference to Admiral Vernon, who was known as Old Grog, after the grogram cloak he wore.
Grog rations were issued until 31 July, 1970, when the practice was ended in the Royal Navy. The US Navy had issued grog rations until 1914, and during subsequent joint operations, American sailors would try to visit British ships when the daily grog ration was being dispensed. Conversely, Royal Navy sailors would head straight for the ice cream machine when visiting their American counterparts.