fist fucking
1.  to enter a body cavity (ie vagina or ass) with one's hand. upon entry the hand is often curled into a fist and repeatedly twisted and thrust in and out of the cavity at different paces and forces.  while pleasurable to some, this activity usually results in great pain and is greatly avoided my most.

2.  disbelief.  due to the uncomfortable connotations of the activity listed in 1, can be used to describe one's outrage or disbelief of a situation.
1.  Markus cried out in pain as Tyrell, the prison gang leader, rammed his hand nearly elbow deep as he fist fucked his very tight ass.

2.  Upon hearing the news that Ann Coulter was actually a man, Michael Moore explained, "you've gotta be fist fucking me! She told me she was using a strapon."