The term "Iron ass" was originally a spacer slang for a Cobra Mk III freighter (the "ass", i.e. a pack animal) fitted with high-end combat equipment (the "iron"), but it eventually spread to mean any ship well equipped for combat. The term was first used in The Dark Wheel.
The traditional Iron ass will be equipped with military lasers, a full quota of missiles and all the defensive and offensive equipment normally available on the market.
For example a Cobra Mk III fitted with Military lasers, ECM, Fuel Injection, Shield Boosters and Military Shield Enhancers, and either an Extra-energy or Naval Energy unit would be considered an Iron ass.
None of the built-in NPCs (non-player characters) will have this level of equipment, so in the normal game an Iron ass Cobra (as above) will be the toughest ship around. As this gives the player a distinct advantage, some OXPs are designed to bring the NPCs in line by making some of them Iron asses as well.
Iron ass ships are generally flown by very experienced pilots, often ranking Dangerous or above.
Many OXP ships are far superior to the core ships in terms of speed, weaponry and cargo capacity. These are normally referred to as uber-ships are not necessarily Iron asses. Of course an uber-ship can also be Iron assed if the right equipment is fitted.