The definition of goony is a slang term that is something or someone stupid, silly or awkward.
An example of goony is the kid in your class that is socially awkward and always trying to be funny and act like the class clown.
Slang awkward, grotesque, stupid, silly, etc.
also sp. goon′ey
(comparative goonier or more goony, superlative gooniest or most goony)