Wikipedia says the term originally was used to describe an axeman serving in a U.S. military unit. “Towards the end of the 19^th century, the phrase was used to describe a Chinese assassin who carried a handleless hatchet…”  The common current use is to describe a person who performs unpleasant tasks such as firing people for a superior. It is also used in sports to describe someone who is given the role of retaliating against opposing players.