The act of simultaneously fisting someone's mouth and vagina / anus. This 'accordian' like squeezing action results in a motion not unlike that of a bodybuilder flexing their pectoral muscles.
If a couple becomes well aquianted with this manouver, and indeed both enjoy it, they may adopt the code phrase "going to the gym". Ie, "I'd love to come to drinks but Sally and I are going to the gym to do some bodybuilding tonight (wink wink)"
Usually verbalised in an Arnold Schwarzenegger accent. Ie, "Da Baady Bill Der"
"Damian is quite the bodybuilder, due mainly to his long arms and soft fists"
"Cherryl's a good sport in the bedroom, but draws the line at bodybuilding"