Literal translation
he's a stitched-up backside
Meaning
he's a stiff, affected sort
Explanation
Someone stiff and affected, a snob who has nothing natural left, down to the most basic bodily functions, as if he'd had his backside sewn shut. The image lands all the harder in a culture that prizes simplicity, where fart and backside are said without embarrassment. Someone rigid is also said to have a mast up his backside (bigue dins lou cuou) or, in Provençal, sèmblo que l'an passa l'àsti (looks like he's been skewered on a spit).