A Window-Licker is a reference to a retard whom only posseses the ability to twist his/her head and lick uncontrollably. Then crossed with the hilarious image of one licking a window.
Can be used in a one phrase sentence I.E 'Lickie-Window!'
Aaron: (retard sounds) Lickie-Window Harry!
Tom: Lawrence is such a Window-Licker! Look at him play cricket!
Alternatives are 'spaz' or 'mongole'.
Mother: You shouldn't be unkind, they are people too
Small child: Eugh! Look at that one windowlicker with the snotty nose!
Mother: Tee hee! They are funny aren't they!