This word is depraved from the saying ''Full retard'' wich was mentioned in the movie ''Tropic Thunder'', where someone made a very stupid movie and a man gave him advice to never go full retard.
In short: Never go full retard..
Guy1: ''Oh my god, did you see John totally act stupid when drunk and get hated by all the girls?''
Guy2: ''Yeah, he's so maxtarded, going full retard is always bad.''
Guy3: ''Maxtard (Full retard mode) is such a lame word, you guys.''