Short for machine. Could be used to criticize another for robotic, repetitive or otherwise useless actions.
Or to phrase someone who is hard-working, well-built or careful
Person 1: Brad just bought another 6 pairs of Js, 4 fitteds, and he doesn't have a job.
Person 2: Man's a buyin machine.
Person 1: No wonder Sarah's always tired and never goes out, she works 3 jobs and 40+ hours a week.
Person 2: Girl's a machine.