Definition of: sheep dog

1. A dog trained to guard and control sheep; shepherd's dog: often a collie, but also a rough–coated, heavy, short–tailed dog much used by drovers in England.
2. Figuratively, a chaperon.
—old English sheep dog A bob–tailed dog of undetermined origin, used as a sporting dog, and, in Great Britain, to herd flocks: characterized by a strong, muscular, thick–set body, covered with a very thick gray, grizzle, or blue–gray shaggy coat.