Disclaimer: Mental retardation is a MEDICAL term. The term "retard" DOES NOT refer to those people who've actually been diagnosed by a medical professional (according to the Diagnostic and Statistical Manual of Mental Disorders or DSM IV) as being "mentally retarded."
(n.) 1. One who isn't diagnosed with mental retardation, but regardless, he/she WILLFULLY REFUSES 1) to act with basic common sense, 2) to display even the most basic form of intelligence, 3) to use logic and reason, 4) to read (quite useful for learning), 5) to speak in coherent sentences.