What is the origin of the phrase "in your backpocket"?
As in "What song have you got in your back pocket?" for "what song have you got ready to perform comfortably now, without preparation".
A quick search of Google Books suggests this figurative phrase is fairly recent.
New York Magazine (28 Jun 1982 - Page 15 - Vol. 15, No. 26) has an article on public speaking:
Maye adopts the Socratic method and teases out of us the idea that answering questions is a welcome way to clarify points and resell your audience. But how to prompt people into asking any?
"Have a transition question of your own in your back pocket," she clues us. "Raise your own hand and ask it."
It's similar to having a "trick up your sleeve" Except, I feel like "in your back pocket" usually implies something physical, where a "trick up your sleeve" is usually more of a non-physical thing.
There's also having a person in your back pocket, which is the meaning that Mynamite was talking about, where you have a person ready to do your bidding at any time. It's not always the result of a bribe, but it often has that implication as well.