big dipper 
noun
1British A roller coaster.But, down below, the Prater was alive, with its booths and rides, the sausages, the beer, the chips, the kebabs and the fairy floss, the ghost train, the big dipper roller coaster....
- While I'm on a rollercoaster tip I think I'll bore you with tales of riding one of the greatest wooden big dippers in the world.
- Those of us successful in the sudden rush for shares in Air New Zealand may be prepared to forgive them for the fact that climactic conditions in the country make their flights more akin to riding a big dipper than to gliding.
2 (the Big Dipper) North American term for plough (sense 2 of the noun).