To shove someone's head by placing an open palm on the person's face and pushing. Similar to a slap, but without the speed and with the force directed forward rather than sideways; typically used to escalate a verbal argument just before coming to blows. It's the most offensive thing you can do to a person without causing them pain.
You can't face-mash a guy without expecting to get punched in the face.
Can you define these popular missing words?