Literal translation
every single time, an eye falls out of him
Meaning
it never happens
Explanation
This expression means that something never happens, through a bodily image pushed to the point of absurdity. Cado còup (every time, còup being a spelling variant of cop, blow, time) sets up the expected repetition, which is then contradicted by the stated consequence : uei (eye) toumbo (falls), from toumba, to fall. Losing an eye every single time is plainly impossible, which flips the expression's meaning into an absolute negation : saying something happens a cado còup li toumbo un uei amounts to saying it never happens at all. The pronoun li, meaning him here, is a very common regional form, modelled on its Provençal equivalent, which can refer to a person, a place, or a moment. The expression is typically used in reply to a claim of every time, as in every single time he tells the truth, an eye falls out of him, mocking the supposed frequency of a fact through the absurdity of the proposed penalty. The register is familiar and outright comic.