A sudden, painful deflation and retreat of the penis, most commonly during an erection, although there are cases of musnis occurring during flaccid states. When a musni occurs, the penis undergoes painful tingles (as opposed to pleasant tingles, such as when your foot falls asleep), and its actions are quite like those a meerkat's when it senses danger - it retreats stealthily and silently into its burrow.
Also like a meerkat's retreat, a musni is triggered by extreme bodily pain and/or fear.
Duke: I saw my grandma naked when I was 5. I've had a musni ever since.
Tyreke: Did you try luring it out?
Duke: It's sorta scared.
Harvey: Did someone say musni?
Tyreke LOOK WHAT YOU DID HARVEY! GOOD GOING, FUCKER.