A fall-guy, a scapegoat. An innocent person who is being framed as a suspect for a crime committed by someone else. A common use of the word is to describe Lee Harvey Oswald as a "patsy" for the assassination of John F. Kennedy - by people who believe that JFK was killed by someone else. It could also mean a person who has played a small part in a conspiracy, but who has been left to take the blame while the main criminals get away.
It may be a corruption of the Italian word 'pazzo' = madman or another Italian dialect word 'paccio' = fool. An alternative theory suggests it may derive from Patsy Bolivar, a character in a 19th century American minstrel act, who was blamed whenever anything went wrong. All of these fit the 'fall guy' idea, but there is no certain evidence for any of them.