Literal translation
he's a stitched-up backside
Meaning
he's a stiff, affected sort
Explanation
This phrase describes a stiff, affected person, socially rigid, of the prim, untouchable sort. The literal image links the word for backside to the idea of stitching, as though that part of the body were sewn shut, made impervious to any ease or naturalness. The same bodily logic appears in French expressions that translate moral stiffness into an imagined physical rigidity. The phrase generally targets a social manner, a way of holding oneself too straight, too reserved, too concerned with propriety, well short of a serious moral flaw: it is said of someone lacking spontaneity and warmth, without necessarily attributing bad intentions to them. The register is familiar and mildly mocking, and the compound word functions as a fixed noun, invariable in form, applied equally to a man or a woman.