What's That Mean?

Something that's been on my mind lately is when I see common words and phrases that have become generally accepted by the public, and how they came to be while the original meaning of the phrase has become archaic or even forgotten over the years.

The biggest one that comes to mind for me is naturally media/film related. Footage. We've all come to accept and freely use the term 'footage' for anything we shoot/film/record/video something. But WHY footage?  Film stock is sold in reels/cannisters of XX feet long.  You don't buy 3 minutes of film, you buy 300 feet of film. The reason why feet instead of times is because if you're shooting 24 frames per second, (fps), you're going to use more film than if you're shooting in slow motion, say 16 fps or stop-motion where you're only exposing one frame at a time between shots. 

So when you're shooting film, you're shooting XX feet of film, or footage.