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When someone jams their face into a window like they're trying to punch the other side. It's usually because they're either being a goofball or they're mad and want to look like a total idiot.
My cousin face-not-glassed the bathroom mirror just to see if it would break.
He face-not-glassed the bus window because he thought the driver was talking to him.
She face-not-glassed the phone screen because she was yelling at her ex.