Let Fly Meaning

(idiomatic, intransitive) To kick or hit a projectile with great force.

Example: 2011 October 29, Neil Johnston, “Norwich 3 - 3 Blackburn”[1], BBC Sport:
  Norwich were back on level terms through an equally stunning finish by Morison in the 53rd minute, the former Millwall player letting fly from the edge of the area after being teed-up by Leon Barnett.