When one clutches their hand into a loose fist as if holding a rolled up piece of paper, and shakes it back and forth in front of someone's crotch. This will make the person feel very awkward, at which point you will have to explain your actions to them in the following way. "See. . . a monkey is jacking you off . .. thats why its awkward!"
When a guy is butt fucking his girl while she is laying on her back. At this time he inserts his hand into the woman's vagina and wraps the thin membrane between the anal and vaginal canals around his cock. This enables him to jack himself off while fisting his mate at the same time.
Oscar loves giving Christina the Velvet Monkey Fist since he loves anal and she loves fisting.
When a man defecates in his hand, then squeezes it so that the feces leaks out from between the fingers, and thrusts it into the vagina or ass of his partner.
Bill: Hey, John, have you met my new girlfriend?
John: I hate to tell you this, dude, but I used to totally monkey fist her.
Bill: That's disgusting. And kinky.