Someone who is stupid, and who doesn't think before they act/speak. Spoonerism of "mentally retarded," and has come to mean someone who isn't retarded, but acts like it.
Dang, that kid just forgot his shoes. He's rentally metarded.
The inability to efficiently utilize the WashingtonMetro, mainly due to unfamiliarity with mass transit systems.
Examples of Metarded including riding the travel several stops in the wrong direction, switching trains at the Arlington Cemetery stop, and frequently missing,"doors closing, step back," by 30 seconds or less.