A scapegoat, one who is blamed for the actions of others. For example, He refused to be the fall guy for his colleagues. This expression uses fall in the sense of "consequences" or "blame," which originated in prison slang. [Slang; early 1900s] Also see take the fall.
An easy victim, one who is readily duped. For example, His friends had marked him as the fall guythey knew he would believe their ruse. [Slang; early 1900s]