Literal translation
he did not invent gunpowder
Meaning
he's not the sharpest tool in the shed
Explanation
The expression signals, with a gentle or mocking irony, that someone lacks ingenuity or quickness of mind. Literally, a pas inventa la poudro translates as he didn't invent gunpowder: gunpowder, a major invention that transformed warfare, serves here as a playful yardstick of human genius, so that saying someone did not invent it amounts to an understated way of saying they never invented anything remarkable and are not especially bright. The form inventa, attested in our sources under the variant enventa (to invent), keeps here the spelling proper to this particular expression. The phrase matches almost exactly its French counterpart, evidence of an image shared between the two languages, and its register remains familiar, used without real unkindness to comment, in a given situation, on someone's slow wit or lack of resourcefulness, in a passing tone.