Finding the meaning of Glasgow kiss was a bit harder than usual. Mostly it was because there are name clashes with poetry, TV Series, drawings, etc.
Wikipedia rescued me. Here is a quote from teh Glasgow kiss page:
A Scottish kiss (sometimes also called a Glasgow kiss) is a British slang term for a headbutt. It is considered derogatory because it portrays people from Scotland as being violent.
And from the Head butt page I learned that it is a fighting technique:
Head butting is a fighting technique that uses the cranium to strike an opponent’s face to cause injury or a knockout.
The head butt is very powerful as the entire body can be driven behind the strike. It is also potentially damaging to the person throwing the head butt. At the front of the cranium are two bony prominences, which the brain can strike into during the delivery of a head butt causing dizziness or even unconsciousness. There is also the risk of shattering the opponent’s teeth and cutting the forehead (HIV infection is a concern).
When delivering a head butt, it is wise to control the opponent’s head by holding the back of the neck. This will keep the head static and easier to strike without injury to yourself. It is also a good idea to strike above the hairline, as any abrasion of the skin will mean the blood coagulates in the hair and does not run into your eyes.
The head butt is powered by bending of the waist and driving with the legs, not by neck movement (which can cause whiplash).
Targets for the head butt include the nose, cheeks, chin, jaw, sternum, solar plexus, and collar bone.
The head butt can also be delivered backwards to an opponent who is holding you from behind.