break (one's) arm patting (oneself) on the back

To excessively praise oneself for an achievement or a job well done. Tony was instrumental in the success of the merger—or so he says. He's going to break his arm patting himself on the back. This was truly a team effort, so no one should be breaking their arm patting themselves on the back about it. Thank goodness they got that guy off the stage. He was going to break his arm patting himself on the back for his donation!