(derogatory) ~ sl.: euphemism for a person of mentally challenged status. Etymology: C20 - Derived from the stereotyped behaviour of mentally challenged people, observed to place their open mouths into prolongued contact with the external windows of public or private transport on which they are seated.
An offensive term meaning a retard, someone who lickswindows.
Comes from when people with learning disablities etc go on coach trips, it is a common belief that they lick windows.