In some cases, I have found it to be true, that the person my hubby and I spoke to each other of when asking each others opinion of a new person we met. The body is alive, yes . . . but when it comes to their mind being in gear, it is not, the person has a vacant look, it used to be called 'spacey' spaced out, a space-cadet, they can't answer intelligently or are not intelligible meaning they aren't making sense so they can be understood. It is best not to use such as phrase because it could be the look of someone on drugs, or a mental patient and that would then be very rude.
I do know of a different version used, only when a person is daydreaming or internally visually something else but not really in the present when a friend comes along, see's you staring into the distance, waves a hand in front of you and says, "Hello, is anybody home?" This then is a friendly tease of the fact they caught you day dreaming and is not meant as an insult. [ Dragonflymagic's advice column | Ask Dragonflymagic A Question ]
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