From Longman Dictionary of Contemporary Englishhired gunhired gunAmerican English informalKILL someone who is paid to shoot someone else → gun
Examples from the Corpus
hired gun• Her coming was more like bringing in a hired gun.• The most free-spending hired guns are all well-known by political mavens inside the Beltway.• This time Bruce Willis is the hired gun in Prohibition-era Texas.• This time Bruce Willis is the hired gun, caught between Chicago gangsters fighting for control of the hooch business. 