a) The stupid or unmotivated feeling one sometimes gets after they come down from a marijuana high.
b) The constant stupid or unmotivated attitude of a heavy pot smoker. A true pothead is always in a state of being burnt out regardless of whether or not they had smoked in the past 48 hours.
See also: burn out
a) "Man I smoked so much reefer earlier today and now I'm burnt out and don't feel like doing shit."
b) 1: "Why does that kid skip class so often and get such shitty grades"
2: "He's totally burnt out dude"
1: "Oh ok that explains it.