In the act of wiping one's bottom, when the toilet paper tears and, inadvertently, a finger plunges into the butt hole. (Occurs especially when using cheap toilet paper or when wiping too hard). Can result in a stinky fingernail.
Guy in toilet: Oh, dude, gross!
Guy outside: What happened?
Guy in toilet: I was wiping and I accidentally did the one-fingered butt plunge.
Mabel found great pleasure in the slow, sensual insertion of a stainless steel butt plug while masturbating. Lucy, on the other hand, having adopted an "exit only" philosophy, was opposed to all aspects of butt pluggery.