Vonnegut never provided an etymology for granfalloon, but it’s not too hard to follow his logic. “Proud”? That’s the gran-. “Meaningless”? Possibly the -loon. This splendidly useful word is so puffed up, so grandiose, so silly — compare buffoon, lampoon, and the Bugs Bunnyism maroon — that it can only mean what it sounds like.
One of my favorite Vonnegutisms. Ting-a-ling, too.