(1) One who is incredibly stupid or did something retarded. Combination of the words 'dumb' and 'pussy' or possibly a shortened 'dumbass'.
(2) Word coined by Strong Bad on Homestar Runner, used to make fun of Strong Sad.
(1) "Jim, stop being a dumpus and hand me that screwdriver!"
(2) "No misspellings + good idea + made fun of dumpus=SOLID!"