A humiliating act, usually
done by your closest friend; when one is "given the goose" so to speak, one
never truly recovers.
The act itself is simple: Sneak up behind an unsuspecting
victim (preferably a friend, as being knocked unconscious is likely to occur otherwise), and as quickly as you can, with all of your fingers on your hand extended out together,
poke them into your friend's buttcrack and run away.
A proper "Goose Goose!" is always followed by you vociferating "Goose Goose!" a split second prior to you fleeing the scene.