A person who is an extra large douche, especially one that makes an ass of himself as part of his general douchy-ness.
Derived from the Latin for hindquarters "Gluteus Maximus," where "Maximus" means "really big."
#1: "So, guess what Douchius Maximus said today?"
#2: "Tell me!"
#1: "He asked again if he could have the vacuum cleaner."
#2: "No! What did you say?"
#1: "I told him no."
#1: "Seriously. He has one, it's just not as nice. You don't have an affair with my friend, move out, leave me with three children, and then ask for the new vacuum cleaner because you took the old one. Kids are messy. You are not. You have a job. Go buy a fucking vacuum cleaner."