noun: A windowlicker is a derogatory, informal description of someone with severe learning disabilities and/or a physical incapacity which renders them helpless when faced with the prospect of seeing a stranger through a window without smearing their mucus covered tongues all over the glass, possibly as some kind of retard greeting.
Alternatives are 'spaz' or 'mongole'.
Small child: Tee hee, look at those windowlickers!!!!
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!