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Definition of The lights are on but nobody's home

Babylon English

expression suggesting and implying that a person is stupid (Slang)
The lights are on but nobody's home Definition from Language, Idioms & Slang Dictionaries & Glossaries
ENGLISH IDIOMS 2.EDITION
the eyes are open but lifeless, not all there """Hello! Hello, Pat! His lights are on but nobody's home. Ha ha."""
English Idioms WM 1.3a
the eyes are open but lifeless, not all there 
"Hello! Hello, Pat! His lights are on but nobody's home. Ha ha."
Australian Slang
someone is not too smart