1. To throw something very quickly and suddenly. I had just given an admittedly snide retort when he let fly with a punch to my head! The audience began letting fly with all manner of garbage and refuse as they booed the unfortunate singer off the stage.

2. To give a very quick or sudden response, especially some critical or sarcastic remark. The student let fly with a comment about the teacher's wife, which landed him straight in the principal's office. The comedian, known for her sharp, merciless wit, let fly with a torrent of off-the-cuff jabs at the host of the show.