Derogatory. A mentally handicapped person, a retard
Possibly derived from the name of Heol Goffa 'Special
' School in Llanelli, South Wales, however its use has spread much further afield.
Its use can be accompanied by a special hand gesture and jib
to emphasise the point:
The jib is accomplished by placing your tongue in front of your bottom teeth and pushing your bottom lip outwards, and crossing your eyes.
The hand gesture consists simply of slapping the back of one limp hand with the other.