Sentence:  He set his teeth, and <WSD>let loose</WSD> with a fury before which nothing could stand; and Maurice was forced back step by step until he was almost up with the wall. 
Meaning of the word marked with <WSD> tags:  or (on) To shout, make a loud sound, or perform a sudden, vehement action; to behave in a raucous, frenzied manner.