2. One of the very funny Lines from the very unsuccessful film Hot Rod
Friend 2: Where do you get off?
2. Rod: Where do you get off?
A rather literal analogy: When on a city bus/train/subway/etc., you might meet some odd person. You might ask "Where do you get off?" as in asking what stop they get off at, wondering if they live in a neighbourhood with strange folk.
Probably not the true origin of the term, but that seems like a pretty good guess if you ask me.
Guy 2: "Where do you get off?"