Literal Translation
Not to have the light on on all floors
Actual Meaning
Lights are on but nobody’s home? Not being the sharpest tool in the shed.
Etymology
The origins of this expression are uncertain and several hypotheses are plausible.
This could be linked to the previous idiom, ‘ce n’est pas une lumière’.
Another likely explanation for “not having the light on all floors” comes from the trivialisation of light produced by electricity, the mechanism of which has been made simple and basic for the population, just like gas. ‘Not having all your lights on’ is therefore a descriptive metaphor for simplicity and is used to refer to someone who is not up to speed with the electrical network, and therefore has a limited brainpower.