The term beast mode  has become common slang, but what does it really mean? According to Urban Dictionary, beast mode is defined as the mode that you switch into when doing hardcore activities; having beast-like characteristics.  Most often, it's used to describe someone who works at high intensity in the gym.

slang. : an extremely aggressive or energetic style or manner that someone (such as an athlete) adopts temporarily (as to overpower an opponent in a fight or competition)