Similar to off your nut, but reserved for extreme cases in which the person has almost no control of themselves anymore - usually due to high drug consumption.
"Dan how much have you sniffed tonight? You're completely off your cunt"
"I'm not surprised you don't remember last night, you were completely off your cunt"
"I'm not surprised you don't remember last night, you were completely off your cunt"
by ZzScotty January 18, 2019