Emperor's Fist was a title used to denote the status of the Emperor's right-hand within the Galactic Empire. Presumably deriving from this, the Imperial Navy reserved a special award, the Medal of the Emperor's Fist, for its TIE pilots.
Since the early years of the New Order, the Sith Lord Darth Vader held the title as the handpicked second in command of the Emperor, his master Darth Sidious, in place of a proper military rank, up until and after his elevation to Commander-in-Chief of the Imperial Forces. As a moniker, it was widely spread, known to both his subordinates and peers as well as to the criminal underworld.