Literal translation
he is good neither for boiling nor for roasting
Meaning
he's good for nothing at all
Explanation
The expression signals that a person is useless, whatever task they are given. Literally, es bon ni pèr bouli, ni pèr rousti translates as he's good neither for boiling nor for roasting: the image comes from kitchen and butchery vocabulary, where certain cuts of meat are meant for the stewing pot while others, finer, are reserved for oven roasting or the spit. Saying a cut is fit for neither use means it holds no value for any preparation at all, an image transposed onto a person judged unfit for any kind of use. The register is familiar and bluntly critical, used to denounce someone's total inefficiency or chronic incapacity to make themselves useful, no matter the task they are asked to perform. The double negative reinforces the exclusion, leaving the person targeted no way out.