Boing! is also the
catchphrase of the
Mr. Saturns from the
Super NES game
Earthbound. The phrase is not merely
Boing!, but it's written in a unique, wacky, game-specific font featuring lots of loops and squiggles.
Boing! is typically used in the middle of a sentence of line of dialogue. Example: "Me Mr. Saturn. Boing! Can do all you for. Boing!"
Boing! doesn't seem to have any specific meaning except to add to the wackyness of the Mr. Saturns. They are the most unique group of characters in Eagleland.