How to Say "empty-headed" in Spanish
The Spanish word for “empty-headed” is “hueco” — C1 level.
English → SpanishC1
adjectiveC1informal
lacking intelligence or substance

Examples
No me gusta su filosofía; me parece muy hueca.
I don't like his philosophy; it seems very shallow (or empty) to me.
Aunque es guapo, es una persona bastante hueca.
Although he is handsome, he is quite a superficial person.
Figurative Extension
This meaning extends the idea of 'hollow' (Definition 2) to describe a person's character, implying they are empty inside, lacking seriousness or intelligence.
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