Green’s Dictionary of Slang

a lunatic, an eccentric.

1983		R. Price Breaks 295: I had to gawk at every whack-a-doo sitting on a stoop.
2008		R. Price Lush Life 304: I’m a whack-a-doo! I’m a whack-a-doo! I got sent to the psycho doctor .
2025		C. Hiaasen Fever Beach 350: [A] whackadoodle who believed that Noah brought dinosaurs on the ark.