the.com/fibrillation

means A rapid, irregular quivering of muscle fibers — especially in the heart — instead of the normal coordinated contraction that actually pumps blood.

from From Latin 'fibra' (fiber, thread) plus '-illum,' a diminutive tag, giving 'fibrilla,' a tiny fiber. Add '-ation' for 'the process of,' and you get a word that literally means 'little fibers doing their own uncoordinated thing' — which is exactly what happens when heart muscle fibers twitch out of sync instead of contracting as one.