A fighting move. Applying pressure with your thumb underneath and behind the jaw. Not effective under the chin. You really want to get your thumb in behind the jawbone and hold the pressure as long as neccesary. Excellent if someone has you in a hold. Less permanently damaging than gouging their eye out.

While performing intercourse "doggie-style" this maneuver involves the pitcher inserting his curled finger into the catcher's mouth and pulling back to degrade the subject or cause pain.
A "double fish hook" involves the same maneuver using both fingers.
A "rusty fish hook" involves fingering the ass of the catcher prior to insertion of the finger(s) into the mouth.

Putting a finger in someones mouth while trying to hurt them and then pulling as hard as you can to rip their cheek. This resembles a fish being hooked so is called the fish hook.
As seen in The Gangs Of New York

After sliding your finger into the cathedral you then curl your finger and pull back.

using your index finger to pull down a girls shirt in order to see her boobs.

The act of grabbing/rubbing a mans package either through the fly of his pants or over the pants. The fish hooker uses one or two fingers, curled in a hook shape to perform the manoeuvre.

When one grabs a women with a claw like hand in the private area.