Se mettre le doigt dans l'œil

Literally: To put one's finger in one's eye

Example: “Si tu crois qu'il va rembourser, tu te mets le doigt dans l'œil.” — If you think he'll pay you back, you're barking up the wrong tree.

English equivalent: To bark up the wrong tree

Origin: The polite explanations reach for someone crossing themselves so clumsily at the font that they poke themselves in the eye — and those explanations are themselves putting a finger in an eye. In slang œil also means the anus, and in common speech error is habitually described with images from below the belt: to be wrong is se foutre dedans, and being deceived by someone draws frankly sodomitical language. Read that way the finger needs no explaining, and the phrase describes deceiving yourself rather than being deceived. Since doigt and œil are now generally understood in their innocent senses — probably for the best — the expression grew intensifiers as the error grows: jusqu'au coude (up to the elbow), or even jusqu'à l'omoplate (up to the shoulder blade).