Originally short for "retard," but this term is not seen as politically correct. Re generally holds a positive connotation, and should be seen almost always as a compliment. The standard-plus-minus scale can also be applied to this helpful term. (RE+ being a good re, RE being a plain old re, and RE- being a re in the negative sense.
When the three sillychildren braided their hair together and galloped across their homeroom, the rest of their classmates shook their heads and laughed, "What res."