Armijello is a contraction of "Arm" and "Jello".
Armijello is the arm wattle displayed by some older people that wobbles like Jello. It is the floppy, loose skin hanging down from the person's triceps area when they extend an arm horizontally. It is the result of the skin having lost elasticity and drooping away from the muscle and bone. Armijello resembles the wattle on certain birds in consistency, hence it's other name, "Arm wattle."
On the road today we were passed by a 70 something grandma riding her Harley. At first I thought she had on a fringed buckskin jacket, but as she passed us I realized she was wearing a short sleeve shirt and it was her armijello flapping in the breeze.