Wacco means crazy.
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The answer would be wizard.
Crazy
It means they have blonde hair.
If you're referring to the point in the book where the boys say, "Wizard, Wacco!" They're not actually calling the island a wizard. This book was written in 1950's England. Just like in America where we might say, "Awesome, Brilliant!" That's similiar to what they're doing here.
It means you are so stupid you watch the 'Lord of the Rings'. Really? That was a dumb question.