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.
Pronounced mee-tard; simply put, an individual whose personal moral and cognitive development has been atrophied; by either self-delusional confidence in one's own abilities, or a refusal to see past one's own tastes and preferences.
Contentious, in that outside of a technical usage this word may be construed as offensive.
A person who ironically proves their own stupidity as they attempt to prove the ignorance of others. The word is condensed from "metatard".
Person 1: "I'm trying to decide where to go on a vacation next year. I'm thinking of something near the Mediterranean. What do you think, Italy or Egypt?"
Person 2: "Do both, I mean.... hello... they're both in Europe... duh..."
Person 1: "Actually, Egypt is in Africa, you metard."