Captain Jocelyn Lucas notes in Hunt and Working Terriers (1931): "Working terriers means, in sporting parlance, a terrier that will go to ground on fox, badger, or otter, and not merely a dog that will kill rats or hunt out rabbits."
In short, dirt work and underground.
Brian Plummer, who wrote several books about ratting with terriers, agreed, using the name “sporting terrier” to distinguish his ratting dogs from those facing tougher game underground.
None of this is to take away a thing from true ratting dogs, especially those who can knock off a dozen rats in a half hour or less. Excellent!
But if your dog catches a rat by the barn, a possum on the driveway or a lizard by the pool, is it a working terrier? No, and for the same reason a man who hits a deer with a Ford truck, or traps mice in the kitchen, is not considered a hunter.
Here’s a simple test: If you don’t own a locator collar, a serious shovel, a digging bar, and a root saw — and you haven’t used them at least a dozen times digging on your dog underground — you don’t have a working terrier.