Used in challenging a person who has said or done something offensive as an equivalent of the expression, "how dare you?"
Where do you get off saying such a thing?
1. (inquiry) Used when someone has crossed the line, in your opinion.
2. One of the very funny Lines from the very unsuccessful film Hot Rod
1. Friend 1: DUDE I JUST TONY DANZA'D YOUR EX GIRLFRIEND!
Friend 2: Where do you get off?
2. Rod: Where do you get off?
A question asked when someone has done something considered to be awkward, strange, and/or just plain odd.
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 1: "So yeah, I like to run around naked with a ball of wool in my hand, and my cat chases me trying to get the ball of wool. It's sooooo much fun!"
Guy 2: "Where do you get off?"
Basically means "why are you...", as in "why are you having sexual relations with my wife, friend?"
Or, as most of us would put it:
"Where the fuck do you get off fucking my wife, asshole!?"