Spinning or Indoor cycling is a form of high-intensity exercise that involves using a stationary exercise bicycle in a classroom setting. A jumbie is a ghost, or spirit of the dead said to possess humans during ceremonies called jumbie dances.
A SpinnJumbie by my definition is therefore that spirit that takes over your body during a spin class.
Note that not everyone can be possessed by the spirit.
Those who are strong enough to resist the jumbie spirit tend to be found in the back row hiding from the instructor and pretending to have tension on while they slowly pedal with no effort at all.
Those people in the front row are unable to defy the spin jumbie and at full spirit possession, they are able to ignore the pain in their legs as they pedal with ridiculous speed.
Some possessed people may even smile and look like they are enjoying the music while the sadistic instructor at the front is torturing the class by inhumanely prolonging every exercise. Some crazy fanatics have even been known to do back to back classes.