1. Refers to the male bicepts usually in an attempt to show them off to the 'lady types'
2. Can also be a metaphor for one's ego and current state of adrenaline or pumped-ness.
(Saying to the lady-folk or man-folk if you roll like that) "Check out these manguns, careful now."
(Saying to someone making you look bad) "Quit steppin all over my manguns."