Definition: To lift weights; exercising to build muscle mass.
Example: 1) On Saturday mornings, Tom is usually down at the YMCA pumping iron.
2) Let’s go to the gym and pump some iron.
Etymology: The weights that a bodybuilder lifts are made of iron or some other heavy material. A ‘pump’ is a device that moves water through repetitive motion. So the phrase ‘pump iron’ describes the act of lifting weights again and again.