When your girlfriend / wife / female so is being a pain in the ass. Basically like you would call somebody who was being a pain in the ass a Twat. But this is saying they are being a Twat to the Max and really need to back off.
1. I was going to go out Friday and play poker with the guys but my girl was all being Twaticus Maximus about us not having enough US time.
2. Your girlfriend is beyond a bitch, shes a twaticus maximus