A Cup cocktail is a style of cocktail, invented in the mid 1800's, which is now fairly unknown outside of England. They were originally a mixture of gin with fruit, liqueurs, and spices, served over ice. Bottles of pre-mixed cup liqueur, to which consumers would add ginger ale, or lemonade, became common.
The modern descendant of these drinks is the English fruit cup and the very popular Pimm's Cup. For historical examples, see also Bishop cup.