Merriam-Webster's Collegiate® Dictionary, 11th Edition
RETARD
I. verb
Etymology: Middle English, from Anglo-French or Latin; Anglo-French retarder, from Latin retardare, from re- + tardus slow
Date: 15th century
transitive verb to slow up especially by preventing or hindering advance or accomplishment ; impede , to delay academic progress by failure to promote, intransitive verb to undergo retardation, see: delay retarder noun
II. noun
Date: 1788
a holding back or slowing down ; retardation , a retarded person