A fall guy is a man, or occasionally a woman, who serves as a kind of scapegoat, taking the blame for something they may or may not have been a part of. It may also be used as a term in comedy for someone who is consistently on the receiving end of negative jokes or a general fool, and may also be used to describe someone who winds up doing the work of other people.
The most widely used meaning of fall guy is an innocent person who takes the blame for a crime. If a bank is robbed, for example, and the robber leaves behind a clue leading to someone who had no knowledge of the robbery, they would be setting that person up as a fall guy. The police might find the clue and arrest the innocent party, taking the heat off of the true criminal.
Related to this usage, a fall guy can be a part of a crime, but left to take all of the blame, allowing the other criminals to go free. This may or may not be done with the fall guy’s consent. If a group of four people know they are about to get caught, for example, one person might agree to go out and meet the police, turning themselves in and taking the fall so that the others can escape. The other three people might also push the first person out the door and then run, forcing the first person to serve as a scapegoat against their wishes.
The third usage of fall guy is simply a general fool who is easily duped. This is often focused to refer to the butt of a joke, especially one who is always an easy target. In comedy writing it is common to create a character who acts as the fall guy, offering an easy avenue for other characters to make jokes.
The least common usage of fall guy is to describe a person who often does the work of other people. This is related to the previous usage of a general fool. The idea of this sort of fall guy is that it is a person who can be easily manipulated or tricked. Huckleberry Finn is a good example of this sort of a fall guy when Tom Sawyer convinces him to paint an entire fence, simply by making it seem like an exciting task.
One commonly given etymology for fall guy is that it was used to refer to boxers who would literally fall down to throw a match. This would be more closely related to the third and fourth uses of the word. There is little to back this etymology up, however, although many people continue to refer to it.
Many people have also tried to relate the phrase to a specific "guy" being discussed as taking a fall. Some have pointed to Lucifer, in his fall from Heaven. Others have looked to Icarus, falling from the sky as a result of his foolishness. Still others have pointed to Adam, who was party to the Fall from grace, resulting in an exile from Eden. While there can be a great deal of speculation as to who the original fall guy was, there seems to be little evidence to tie the origin of the phrase to any one person.
Much more likely is that the term fall guy relates to a slang use of the word "fall" from the 19th century, in which "to fall" was simply to be arrested. This would make the etymology of fall guy as simply the guy who gets arrested, which is simple and concise, although perhaps not as glamorous as tying it to Lucifer’s fall from Heaven.