When you trick someone into thinking that somthing horrible has happened and then you litterally poke them with a fish and say "fish poke."
"Uh oh Tommy!"
"What is it Bobby?"
"(pokes Tommy with fish) Fish Poke!"
to poke a girl in her ass, generally to her enjoyment.
last night I was massaging Naomi's ass and I was spreading her cheeks, and she was just craving a good fish poke, so I gave it to her. And she said "oooooooooo".
For origins of this term, please refer to the Dutch version of the "Barbie girl" song, it is at the very end.