Frobscottle is a drink found in Giant Country, in "The BFG." It is the only known drink to be consumed there. Although there is water for washing, it is unfit for drinking.
Frobscottle is a green-colored fizzy drink, where the bubbles go down instead of up. This is the cause of Whizzpoppers (which are farts). It tastes delicious to Sophie and The BFG and to the other giants as well. It also makes the drinker fly and fart.
The BFG (Book)
The BFG is the first and only Roald Dahl story to feature Frobscottle. Sophie and the Big Friendly Giant both had frobscottle when Sophie asked if there was water in Giant Country. She thought it tasted of Vanilla and Cream, with a faint trace of Raspberries.
In Roald Dahl's Revolting Recipes, there is a recipe for Frobscottle. You make frobscottle by mixing soft drink (most likely Cream Soda judging by what Sophie thought it tasted like), green food dye and a bit of water.
The Animated Feature
In the Cosgrove Hall-produced animated feature, Frobscottle is purple instead of green. The mouse drinks some and goes zipping about like a rocket.
The props people of the 2016 film made this drink for the BFG. In The Movie BFG Frobscottle is pointed toward being made from snozzcumbers. The Fleshlumpeater steals a bottle for himself after telling the BFG off for waking him up by jabbering.