The lights are on but nobody's home!
Barry: I asked John to finish the report by today but he's not even started it.
Cindy: You'll have to remind him. He forgets everything. The lights are on but nobody's home!
The lights are on but nobody's (or no-one's) home - this English idiom means that you think someone is stupid.
It is a different way to say it.... :)
It is. There are actually many ways to say the same thing. Do you know any others?
This idiom make me smile.... it seems to be a nice way to say it with humour, I don't really know others in English. Is it impolite to say that or it is something just for a kind of nice humour...
It's usually used in a humorous way. Not too serious.
We could also say - He's not the sharpest tool in the shed.