Gamekeeper’s thumb is essentially synonymous with Skier’s thumb, although the latter has a more acute injury connotation.
It was first described in, you guessed it, gamekeepers by Campbell in 1955. Repetitive breaking-of-necks of small game (rabbits and such) resulted in chronic injury to the ulnar collateral ligament of the MCP joint of the thumb. Strangling small furry animals having fallen by the wayside as a recreational activity, and having been replaced apparently by skiing, this injury is seen more frequently due to the stock and stock-strap forcibly abducting the thumb.

Treatment depends on classification, but essentially boils down to: a) if displaced fracture, or b) unstable in flexion, requires surgery.

For classification, and a picture of a rabbit, visit here.
Credit: Dr Frank Gaillard