The light is on but nobody is home 

A common phrase used to refer to someone who's perception is very, very slow. An imbacil for example. Someone who just can't understand.

A: Excuse me, what time is it?
B: Time for you to go home and play with soap!
A: But my mom will be angry if I waste soap?!
B: The light is on but nobody is home
C: Kalevet!!!

The light is on but nobody is home by wandman September 1, 2005

the lights are on, but there's nobody home 

Someone who is mentally vacant. "Fit in body but sick in mind" as one may have said in an ancient Catholic will.

Q:Why does she just sit in that rocking chair all day?


A:The lights are on, but there's nobody home.

the lights are on, but there's nobody home by Kung-Fu Jesus April 21, 2004

lights are on but nobody is home 

Used when describing a person who is not all there mentally, that is inept socially etc, or appears to be a bit thick. Struggles to fit into normal society, and subsequently rejected and described to people as a bit slow.
May also be used lightheartedly if someone is perhaps day-dreaming and not paying attention when asked a question. Similar to, but not quite the same"a few beers short of a six-pack", or "a few screws loose."

What if we asked Dudley if he knows?
What? You're kidding me right? Theres no way he'd know. In fact stay away from that bloke
Why's that?
You know, The lights are on but nobody is home.
Oh, ok thanks for the warning.

I can't remember, hang on a sec, i'll ask Harold.
Harold, whats the price of that Xbox again, you know the one you bought from E-bay?
Huh?
I said, how much was that X-box you got from E-bay the other day?
Oh? umm, ahh, hang on a sec
Sorry for the hold up Mavis, old harold here is in another world, the lights are on but nobody's at home, heheh.

lights are on but nobody is home by jamie_ledge July 19, 2006

The lights are on, but nobody's home 

1. Literally, when someone leaves their house and forgets to turn off their lights.
2. Joe Biden

1.
John: hey, let's go across the street and drop in on Bob.
Jim: I don't think he's home right now.
John: but his lights are on
Jim: yeah, but I saw him leave a while ago, he must've forgot to turn off his lights when he left again, he does that a lot.
John: damn, I was really looking forward to having a few beers and some great conversation with him!

2. Joe Biden is literally the definition of 'the lights are on, but nobody's home.' I mean he has a brain, (or wait does he?), I mean at least I think he has a brain, but we never see it used and it doesn't work properly most of the time, he's alive and here physically but mentally he's dead and gone.

The lights are on, but nobody's home by Cornpop was a bad dude July 20, 2022

lights are on but nobody is at home 

It means you are stupid.

She is not a very bright girl, the lights are on but nobody is at home.

lights are on but nobody is at home by dannetobbe October 21, 2015