Like getting punched in the mouth. Or getting hit by the broom's handle when that one guy in front of you, who's carrying a broom, suddenly stopped.
The previous answers show that there are many words that mean to be hit in the mouth. The correct word depends on the object hitting the mouth.
For example, one could say, "I was hit in the mouth by a mop." A large percentage would think you were hit by the braided cloth end. A smaller percentage would think you were hit by the wood tip; and, a still smaller percentage would think you were hit by the body of the handle, like a baseball bat.
Now, to imply a certain attack, "I was slapped in the mouth by a mop," would more likely create the vision of a wet mop slapping and wrapping around the face."
"I was poked in the mouth by a mop," increases the likelihood of the reader imagining getting hit by the end of the wood handle.
To imply a thwack with the shaft, there are several colorful words. "I was (thwacked, high sticked, Babe Ruthed, beaten like Joe Pesci in Casino) in the mouth by a mop."
The English language does not have a great word that specifically defines getting hit in the mouth, though the German language has one much closer 'Backpfeife'.
In English, I would always use the word socked (depending on the circumstance and audience of the piece being written); "He was socked hard enough in the lips to split them wide open".
Although GOBSMACKED often refers to being utterly astonished or astounded, the online Urban Dictionary does give one meaning as getting literally hit in the mouth. Their example is: "I gobsmacked that bully with my biology book."