Tazviona Maluge Bhebe, 40, stormed into an off licence in Sutton, South London, armed with a kitchen knife.
He was wearing a pair of light blue boxer shorts over his face, peering through one of the leg holes as he threatened 34-year-old shop worker Harry Mahalingham.
A court heard that Bhebe, who lived near The Broadway off licence, demanded cash from Mr Mahalingham and tried to open the till to steal cash after reaching behind the counter.
He told Mr Mahalingham: "Give me your money or I will use the knife", it was claimed.
But the burglary was foiled when Mr Mahalingham jumped over the counter and beat Bhebe over the head with A-frame display sign, causing the boxer shorts to slip down and expose his face on CCTV.
Bhebe chased the shop worker around the store brandishing the knife but eventually ran off, throwing away the knife and ripping the failed disguise from his face.
Police arrived at the scene within minutes and arrested Bhebe after watching the CCTV footage and extracting DNA evidence from the knife and underwear.
Detective Constable Andy Thornton, who revealed a string of CCTV images of the bungled January 21 raid, said: "The CCTV shows his boxer shorts shifting to reveal his face during the robbery."
Bhebe pleaded guilty to robbery and possession of a knife and was jailed for three years two weeks ago at Croydon Crown Court.
A Metropolitan Police spokesman said: "The man walked up to the counter and peered out through a leg hole in his plain light blue shorts and demanded: 'Give me your money or I will use the knife.'
"The CCTV shows his boxer shorts shifting to reveal his face during the robbery"
"As Bhebe walked around the counter to take £100 from the till, the sales assistant quickly jumped over the counter and went outside the shop to pick up a large, A-frame display sign.
"The assistant then returned to the front of the counter where he swung the green sign at Bhebe who was still behind the counter. Bhebe was hit with the sign.
"Bhebe chased the sales assistant outside the shop to the nearby roundabout before running off discarding the kitchen knife and removing and discarding his boxer shorts as he went in full view of a number of pedestrians.
"Officers were quickly on scene and seized the shop's CCTV, secured forensic evidence and took witness statements from the sales assistant and pedestrians nearby.
Mr Mahalingham's boss Ruba Kumar, 46, said: "I've always told my workers not to react in a robbery but he told me he did not want to see any of the shop's £5,000 stock taken so fought back.
"I'm really grateful for what he did."
