Other forms: wackiest; wackier
Something that's wacky is weird or nutty or silly. Your goofy uncle who wears a funny hat and does magic tricks at the dinner table is wacky.
Use the adjective wacky when you're talking about someone with a ridiculous sense of humor. Clowns are, by definition, wacky, for example. Most people have at least one wacky relative, and some are lucky enough to have entire wacky families. Wacky comes from the nineteenth century British slang term whacky, which arose from whack, specifically the idea of getting whacked on the head so hard that it turns you wacky.