The word retard was first used in 1489, but back then it meant delay, stemming from the French word "retardant".
(According to etymonline.com, it was used in the sense of slow childhood development "mentally slow," from 1895 and around 1970 it took on the more offensive meaning "retarded person," in AmE. Now there's a campaign to have it removed from the English language called "The 'r' word campaign", http://therword.org/ spearheaded by Sarah Palin of all people. Good luck with getting it removed from the Urban Dictionary Ms Palin.)