a. a middle aged mentally retarded person.
b. a person who is stupid, obtuse, or ineffective in most ways: socially inept, asks ridiculous or irrelevant questions, requires guidance in the most simplistic of tasks.
(a) That guy is such a leetard, how has he survived so long in life with no idea about anything?
(b) I can't believe I had to explain myself four times! Wat a leetard.