Pirates' potation (4)
(Other definitions for grog that I've seen before include "Nautical drink" , "Mixture of spirit and water" , "Drink of spirits mixed with water" , "Booze" , "Old naval drink" .)
(Other definitions for rum that I've seen before include "Queer" , "Odd sort of spirit used in grog" , "Odd spirit from sugar cane" , "tipple" , "Unusual" .)