literal translation: your sister's pussy! this sentence in English language is not complete because the adjectival is missing. Howerver, in Arabic it is a complete swear word which means: you are a brother of a whore sister. The closest equivelant in English is Mother fucker kos ommak.
In Arabic culture it is very offensive to mention someones mother in a rude way, let alone his sister. The female members of ones family represent his honour and dignity.
Kos ukhtak manyookeh : your sister's pussy is fucked.
When someone is getting less concentrated, confused, or committing mistakes all the time then he is losing his marbels. It is related to the mental status of this person. It does not mean he is metally ill though.
I can not pass this exam. It seems I am losing my marbels
Stop thinking of her, you are losing your marbels
If I stayed unemployed any longer, I will lose my marbels