Literal translation
every time he tells a truth, one of his eyes falls out
Meaning
every time he tells the truth it costs him dearly (said of a liar)
Explanation
The saying mocks a habitual liar by imagining an absurd physical consequence tied to how rarely he tells the truth: telling the truth would come so unnaturally to him that an eye, a precious and irreplaceable organ, would fall out of him every time a sincere word slipped past his lips. This grotesque hyperbole underlines, by contrast, how frequent lying is for the person in question, truth becoming such an exceptional event for him that it would cost a mutilation. The saying is quoted to mock someone known for distorting facts or telling unconvincing stories, in a frankly comic and unsparing register, where the exaggeration serves precisely to laugh at the liar's bad reputation. The word cop means a blow or a time, an instance marking the repetition of the occasion, while toumbo (falls) applies to the human body a verb usually reserved for falling objects. This phrase belongs to the register of mocking proverbs which, in Provençal tradition, like to attribute disproportionate reactions to the body in order to poke fun at a moral flaw.