The open-mouthed facial contraction of one being penetrated, most likely in the anus, caused by the pain of penetrating a tight orifice. Usually he/she will not emit sound though, as they are in too much pain.
Often used in a derogatory manner, as expressing a desire to inflict pain on another.
That bitch Kira is so annoying. I wanna fuck her in the ass just so she'll shut up. Give her the silent scream.