Balls out is a slang phrase used to describe having to put in an immense effort. When one tries very hard, to the point of exposing one's self to risks, then that person has their balls out. This modern interpretation of the phrase visually represents exposing the scrotum to risks. It may be used to describe any gender.
The phrase “balls out” actually does not derive from the testicles being exposed to danger. It goes back to the days of steam engines. The speed of Watt style steam engines was controlled by a centrifugal governor. At fast speeds, the balls of the centrifugal governor would rise out. At full speed, these balls would be completely out. Thus, the term “balls out” was born.
