Literal Translation
They’re not a light
Actual Meaning
They’re not the smartest, not the sharpest tool in the shed
Etymology
Dating from the 19th century in reference to the Lumière brothers, who were undoubtedly ‘des lumières’ for having invented cinema.
A person perceived as ‘intelligent’ is normally and often capable of shining in society, and if they ‘shine’, it is metaphorically that they produce light. If they are intelligent, it is thanks to the light they produce that they can ‘enlighten’ others on many points. So, in this first hypothesis, the term “light” would refer to intelligence, and the verbs “to shine” and “to illuminate”, which were in use in the 19th century, would both be linked to wise or intelligent people. At the time, people spoke of someone’s ‘enlightenment’ to refer to their high intellectual abilities.