Jubnuk is one of Jabba the Hutt's loyal Gamorrean guards. When Luke Skywalker arrives to rescue his friends, he doesn't realize he's standing over a trapdoor that we'd seen used earlier. When negotiations with Jabba fail, Luke, in a last ditch attempt, uses the Force to summon a blaster pistol from a guard's holster, but as he aims at at Jabba, Jubnuk here grabs him, forcing his arm up.
At the same time, heedless of his guard, Jabba activates the trapdoor, dropping Luke through it. Poor Jubnuk's reward for saving his boss' life is that he loses his balance and falls in after the young Jedi and they end up inside the pit together.
"Silly Jedi!" Jabba chortles as he prepares for the show. "Silly Jedi!"
The door at the other end of the pit slowly slides open and disgorges and enormous reptilian beast.
"Oh no!" cries Threepio, ever handy with exposition. "The Rancor!"
Jubnuk's response to the creature is swift and immediate. He begins squealing in terror (Gamorreans are kinda piglike aliens) and starts clawing at the grating covering the chute he and Luke slid through.
All his squealing is attracting the monster's attention, and amusing his fellow Gamorreans up top who laugh and slap each other on the back just to show what a den of vile sadists Jabba's palace is. Even Jubnuk's own fellow piggies are having a good laugh at his expense.
Luke, for his part, is more concerned about himself, and does nothing to help his enemy as the Rancor bends down and grabs the squealing guard up and stuffs him into his mouth. Crrrrrunch!
Jubnuk's cries stop abruptly and the cruel Jabba just chuckles up a storm as he watches the underling who saved his life a minute ago get eaten alive.
Swallowing Jubnuk down, the Rancor turns and eyes Luke hungrily. Next course!