Literal translation
they would make him believe hares lay eggs
Meaning
you could make him believe anything
Explanation
The expression describes someone extremely credulous, ready to believe any claim, however absurd. Literally, ié farien crèire que li lèbre fan d'iòu translates as they'd make him believe that hares lay eggs: the image rests on a biological impossibility known to all, since the hare is a mammal and does not lay eggs, unlike birds or reptiles. Claiming that someone could be made to accept such nonsense underscores how absent their critical judgment is, or how great their naivety runs. The register is familiar and deliberately mocking, used to tease someone easily fooled, credulous enough to check or question nothing. The expression belongs to a rich Provençal family of animal based comparisons built on biological absurdity, whose comic effect lies precisely in the gap between the real natural world and the claim one pretends to make them swallow.