Fluency has to do with "flowing forth" as something coming out. Something that's fluid.
"Fluency (also called volubility and loquaciousness) is the property of a person or of a system that delivers information quickly and with expertise."
I speak the English language effortlessly. I can and hear and understand the English langauge completely as long as there are no other outside or background noise that may obscure it (e.g. listening on the phone versus listening in a noisy area).
