1. The main villain from the movie Road Warrior. A frighteningly muscle-bound man in a strange (semi-nude) executioner-style getup, mostly balding wispy hairs (presumably due to radiation exposure), and speaking in a deep Arnold Schwarzenegger-esque accent. Possibly in homosexual relations to right-hand-man Wez, but don't you dare make fun of him. Nobody defies the Humungus. NOOUBODY! Well, except Max...
2. Alternately used to describe anyone overly intimidating and/or badass, but a bit quirky behind the scenes and possibly with a "touch of the lavender."
Something very large, big or gross. The most popular reference for it is the large, semi-naked, bald baddie called 'Humungus' in the film Mad Max 2 or, in the US, Road Warrior 2.
'Did you see the humungus crater on the waiter's face?'
'Melvin's feet are humungus!'