The act of "accidentally" slipping an ecstasy, or thizz pill onto one's ass during anal sex. One would place a thizz pill against the receiver's asshole without the other knowing, then immediately after- shove his dick, or finger, in this persons anus hole. This requires precision and a loose ass, or a lot of lube. Can be used on a guy or a girl, or a cat. Whatever you like to fuck will be surpised when the sex feels better than middlefuck's tongue in ur ass.
I'm gunna pull a mystery plug on Mario because I want him to let me rail his bungsnacks all night.
Flip is thizzin because I mystery plugged his asshole.