Large Doughnut
The large doughnut can mean mean many things depending on the context.

However in this context it merely means an oversized doughnut, with the added capabilities of flattening things, e.g. a chav. And that's always a good thing.
Person 1: "Hey there's a large doughnut rolling down that hill."
Person 2: "It flattened that chav!"
Both: "Yay!"