to goose is am Po grapschen

the hell out of (something, someone) means to do a lot of it.

Dann grapschen er dir am Pa was das Zeug hielt.

Besseres Deutsch wäre vielleicht:
"Dann würde er Dir ohne Ende an die Wäsche gehen"

Does that mean to squeeze someone´s ass?

to goose someone

It doesn't mean to squeeze (am Po grapschen) or "an die Wäsche gehen". It is, I fear, a cultural specialty probaly limited to Anglo-Saxons.
You make a fist with the thumb pointing upward. Then you ram said thumb into the victim's anus. That is goosing.