When you wrap your hand around your penis and have sexual intercourse while your hand is still on your dick. Your entire fist and dick are being inserted sexually until completion.
Bonus tip: while fistucking you can jack yourself off while inside of your partner to finish faster or last longer.
A "Fishfucker" is a celebrity who is or feels as though they should hide a specific aspect of their lives from the public.
The term is derived from the Simpsons episode "A fish called Selma" , in which Troy McClure states that he has a sexual abnormality towards fish. Also known as the one where Selma marries Troy McClure.
Some examples include the above mentioned Troy mcClure, and several celebrities that have been caught doing terrible things. Although please keep in mind that being a celebrity that is a shithead does not make said celebrity a fishfucker, it's that said celebrity is attempting to hide something from the public at large, is more than likely ashamed of it as well.