(humorous, derogatory) To be very full of oneself. Often used as a sarcastic caution - "don't break your arm patting yourself on the back"
We're the best football team in the state! - Careful, dude, don't break your arm patting yourself on the back.
Origin of break-one-s-arm-patting-oneself-on-the-back
- From pat on the back, and the uncomfortable position one would have to assume while patting their own self on the back.