Popularized by the Valley Girl movement in the San Fernando Valley.
Literally means you need to put a bag over your face, because you either need to shut up, or you're ugly.
Look at your nails and your lipstick, bag your face, please!
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If you are talking slack or annoying someone, they would probably tell you to go and bag your face. The meaning of it is to shut up or be quiet.
Liam: Are you that bitch everyone keeps talking about?
Sam: Bag your face.