The Greaser (other names: M3 Submachine Gun, M3 Grease Gun) is a weapon used by many Splicers in Rapture.
When Frank Fontaine began to smuggle 'legitimate items' into the city of Rapture, Rapture's security officers would sometimes capture some of the cargo before it even got into the city. To fix this, smugglers began to arm themselves with M3 Grease Guns; which were realitively cheaper on the surface than the Thompson Submachine Guns. This proved to be a successful idea, since the M3's could be modified exstensively and were often easily repaired.
During Rapture's Civil War, many Splicers on both sides got ahold of this gun (since they had found secret warehouses full of these guns). When they began using it on each other, many commented on how it "greased" the other side, which then gave the gun the term "The Greaser". After the collapse of Rapture, many of these guns were still found around the city.