sticking M&M's up your balls for sexual pleasure, then letting your partner eat them off.
" Damn, she M Bagged me GOOD last night"
A scrotum with a letter M on it. Usually seen on extremely fucked up mario queers.
Guy: *fucks Girl hard*
Guy: *jizzes in Girl's pussy*
Girl: *looks at Guy's M bag* Omg, I had sex with a fucking nerd!
Guy: Oh shit! *teleports away like a...wierd...person?*
Girl: What the fuck?