A low-level thug used in a fight mainly to increase numbers; an expendable no-name criminal; an unknown.
The concept of mooks is used in many action movies (particularly in kung-fu) and beat-em-up arcade games, where the hero(es) cut through wave after wave of enemies easily (these being mooks) until they reach the boss of the scene, who despite being possibly weaker-looking than the mooks, is usually much more powerful/elusive.
"Remember the Stormtroopers from Star Wars? How hundreds of them couldn't hit the broad side of a docking bay door? What a bunch of mooks."