A sex partner who is either not receptive, or not providing any emotional or physical feedback during copulation.
Perhaps something was on his mind, but the cold fish watched while I did all the work.
When you hold your hand in a "karate-style" fashion, then, with a ramming motion, jam it up an unsuspecting person's ass crack. (Optional) Then yell "COLD FISH!"
John was unpleasantly supprised when I gave him a Cold Fish in the bowling alley parking lot.