A WOULD-BE thief held up a jade shop yesterday - and was beaten and robbed by the have-a-go shopkeeper and his wife. The bungling bandit lost his watch to his enraged assailants, while his accomplice looked on in disbelief. Even firing a ''shot'' didn't help the robber as blows rained on his head. The feeble pop from a fake gun only angered the Mongkok shopkeeper - who chased the terrified pair down the street. Last night, a team of detectives were investigating the stick-up on Shanghai Street, with the thief's watch as Exhibit A. Police said the unsuspecting gunman tapped the shopkeeper on the shoulder, showed him a pistol in his waistband, and said: ''You know what to do.'' The shopkeeper certainly did - the bandits escaped empty-handed after dodging down a lane near Reclamation Street and Prince Edward Road.