1 general:: Lift weights, as in She's started pumping iron three times a week. This idiom was born with the late20th-century stress on physical fitness. [Second half of 1900s]
American Heritage Idioms
2 general:: Phrase(s): pump (some) iron Sl. to lift weights for exercise. • Andy went down to the gym to pump some iron. • Mary’s hobbies are pumping iron and running.
McGrawhill's American Idioms And Phrasal Verbs