Omni-piss refers to the activity of drinking in extreme excess and the full immersion in aspects such as, location of piss, company in which you are on this piss, substance both legal and illegal, and unequivocal dedication to the overall cause.
Slouched and hungover, Sebastian turns to Bloomers and remarks:
"Last night was remarkable, truly an Omni-piss"
Bloomers concurred affectionately and then proceeded to vomit into the sink for the remaining sunlight hours