Ça ne casse pas trois pattes à un canard

Literally: It doesn't break three legs of a duck

Example: “J'ai enfin vu ce film dont tout le monde parle… franchement, ça ne casse pas trois pattes à un canard.” — I finally saw that movie everyone's been talking about… honestly, it's nothing special.

English equivalent: Nothing to write home about

Origin: Absurdist arithmetic: a duck has two legs, so breaking three of them is not merely difficult but impossible, which is what makes the thing being described so unremarkable. The expression is early 20th century, and beyond that even the specialist etymology sites offer almost nothing — no date attached to a named source, no documented first use. A second, weaker theory drags in a cagnard, old slang for a horse with deformed legs, hard to ride and liable to injure its rider — but it comes with no citation either. This is one of the rare cases where the honest answer is that nobody has actually traced it.