»To fight like Errol Flynn«; a term of various (albeit limited) application. Originally it referred to the specific, exuberant and hazardous way in which Flynn would personally stage-fight, which constantly endangered his fellow actors; later, it came to apply to the stage fencing of that era of film more generally: the arming sword gripped and plied in a manner informed entirely by sport fencing, the fights out of measure, the pirate halves, and so on. Ironically, Flynn was well aware of correct fighting distance, and staying in it was one of the primary means whereby he recklessly imperilled his counterparts.
Finally, in the field of WMA, »Flynning« refers to fighting like an imbecile, specifically by the method of acting like whatever weapon you're holding is a sport épée. Oddly enough, in my personal experience this (I cannot stress this enough) thoroughly idiotic way of fencing has been consistently effective when fencing with a sabre, to an even incredible degree. I have no explanation for this.