break one's arm patting oneself on the back

• dislocate one's shoulder patting oneself on the back
• hurt one's arm patting oneself on the back
• hurt one's shoulder patting oneself on the back
• hurt one's hand patting oneself on the back

From pat on the back, and the uncomfortable position one would have to assume while patting their own back.

break one's arm patting oneself on the back (third-person singular simple present breaks one's arm patting oneself on the back, present participle breaking one's arm patting oneself on the back, simple past broke one's arm patting oneself on the back, past participle broken one's arm patting oneself on the back)

1. (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.

• (be very proud): toot one's own horn