A person who hesitates or refuses to proceed with an action or decision. The balker in the group delayed the project by constantly questioning every step.
In baseball, a player, typically a pitcher, who commits a balk, which is an illegal motion or action. The umpire called out the pitcher as a balker after he made an illegal move on the mound.
An animal, especially a horse, that suddenly stops and refuses to proceed. The cart was stuck because the balker refused to take another step.