Ce n'est pas le couteau le plus aiguisé du tiroir.*

* It is not the sharpest knife in the drawer.

French French speaking countries

Expression USED On Rare Occasion BY Some People

It means that you are not really smart but it is said in a funny way.

" - Je n'ai pas réussi à faire cet exercice. - Tu n'es vraiment pas le couteau le plus aiguisé du tiroir..."

"I did not manage to do this exercise. - You are definitely not the sharpest knife in the drawer..."