Literal translation
he is as frank as a donkey backing up
Meaning
he's as honest as a donkey backing up (not honest at all)
Explanation
This ironic expression says exactly the opposite of what it states : it calls someone hypocritical by declaring them frank through an absurd comparison. Franc (frank, loyal, sincere) is asserted at face value, but contradicted by the comparison itself : ai (donkey) que reculo (backing up, from recula, to back up, also to deflate figuratively) describes behaviour opposite to frankness, which implies facing forward and owning one's words, not retreating backward. The image of a donkey backing up, an animal reputed to be stubborn and unpredictable, reinforces the idea of evasive behaviour, the reverse of what is expected. This form of irony through contradictory assertion is a classic device of the Provençal saying, which often favours understatement or paradox over direct criticism. The register is familiar and readily mocking, used to expose, without seeming to attack head-on, the hypocrisy or duplicity of someone who claims to speak the truth while actually dodging it.