"Over"heard

Asian couple, handsome each, in Walmart yesterday. I would say they are Japanese. Whatever. He's a bit impatient, asks the time. "Nearly three," she says. Then she says, "We can go soon." 

He, "Bonanza's over." (It's over at three.) And I realize ME TV, with its old show reruns--some of them pretty darn good, though Bonanza isn't one I watch, being on in the daytime as it is--has a varied audience. Why did I not expect him to be a Bonanza watcher?

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And now my question:

The word "over" seems to have little to do with starts and finishes. When and how did it come to be that we say "it's over" to mean something has ended?  

And then there's "I'm over you."