Final answer:
The combining form fibrill/o- in the medical phrase ventricular fibrillation means twitching or fibrils.
Step-by-step explanation:
In the medical phrase ventricular fibrillation, the combining form fibrill/o- means twitching or fibrils. This is derived from the Latin word 'fibrilla' which refers to small fibers. Fibrillation is an abnormal and uncoordinated contraction of muscle fibers, in this case, the muscle fibers of the ventricles of the heart.