Bhebe threatened shop worker Harry Mahalingham, 34, with the knife and demanded money, but the clerk instead jumped over the counter and struck the robber with an A-frame display sign, knocking the boxer shorts from his head.
Bhebe chased the worker around the store with the knife and quickly fled the shop.
Police said they identified Bhebe from security camera footage and DNA from the discarded boxer shorts.
Bhebe pleaded guilty in April to robbery and possession of a knife. He was sentenced by Croydon Crown Court to three years imprisonment.