– to test ideas, to talk about your idea in order to get sb’s opinion.
I’d like to BOUNCE my ideas OFF you before the conference. I BOUNCED my idea of running my own business OFF of my friends.
I believe it’s associated with the way a ball bounces off something, when you throw it, and gets back to you. You “throw” your ideas to somebody (your friends, workmates, boss) and they “return” them to you with their feedback.