Hired Gun Definition

hīrd
noun
A person hired to kill or defend someone.
Webster's New World
One hired to fight for or protect another.
American Heritage
A mercenary soldier.
Webster's New World
An expert contracted to solve a specific and often difficult or unpleasant problem.
Webster's New World

A person who is employed as an armed guard, enforcer, or mercenary and who is prepared to use gunplay or similar violent methods in order to assert the interests of his or her employer.

Wiktionary