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.