I’ve googled this one - found it in a book I’m reading - and it seems to roughly mean not being very bright. However, I’m puzzled about where it came from and the actual phrase (in a Dramatis Personae list) was " on ne lui demande donc pas d’avoir inventé la poudre", which seems to be a variant?
I’m finding this list of people with mini-descriptions to be rather interesting. The next person was described as “n’a pas pour habitude de garder les yeux dans sa poche” which is also a new one on me…
It’s like ne pas avoir inventé le fil à couper le beurre. It is gunpowder though not just any powder, so if you say it, it means you aren’t expecting much going on upstairs from that person, they aren’t brilliantly clever and that’s ok.
Usually it’s your tongue you don’t have in your pocket rather than your eyes, so maybe a malapropism or a variant for comic effect.
Your book sounds a hoot!
Oh just thought, we also say bon allez on ne va pas réinventer le fil à couper le beurre when we mean use information or skills and put them in common.
It is! In order to cheer myself up about my level of French I decided to replace my lost copy of a favourite English book with a French translation, since I already know the plot and have a good chance of feeling comfortable with it. It’s the first of the Lindsey Davis Falco books - bit lightweight for you perhaps? 
Thank you for the explanation of the terms - it’s good to have a fuller explanation than the dictionaries give!
Oh no I love the Falco books!! Such fun and also so well researched - in a similar vein but for children I loved all Rosemary Sutcliff’s books but especially the eagle of the 9th and the silver branch 
Ooh I would give you TEN likes if I could. Adored her books.
Forgot to say I think this is a fantastic method for getting your foreign languages up to scratch 
Thank you @vero . I should have been doing more of that as an earlier French teacher encouraged me to get translations of authors I liked and I found a wonderful early Margaret Atwood book in the local library here. I hadn’t read it before but was competely hooked and it helped no end.
A bit of a digression but I was recently told an expression that I think is rather lovely. A French friend I have known almost since I arrived in France is retiring to a different part of France, we have gone through the ageing process together and I will miss him being around, and as we were saying our farewells he taught me the saying “Il n’y a que les montagnes qui ne se rencontrent pas” or in other words, there is always a chance that we will meet up again one day.
yes, it’s sad when a good friend moves so far away… I always feel gutted.
On the other hand… there is now a new place for you to visit and a very good excuse for making the trip… 