Why do we use the terms "fast" and "slow" for foca

When an instrument has a high focal ratio we call it "slow" and when an instrument has a low focal ratio we call it "fast". Why?

Hugh, does that mean there's less light reaching the eyepiece of the higher f-number? I know there's less light when you stop down a camera. If there is less light, does that mean you can't see as much, or is it more like squinting and you just can't see as wide of a field of view? Or is it both? So is a higher number f-value telescope better for photography and worse for viewing?

Thank goodness for a "beginners" forum where one can ask such questions.

Bob