On: Casu Martzu

[C-x_C-f]

From what little I know of Sardinian, I'm guessing su caggiu simply means the cheese?

[funebre → C-x_C-f]

No, but you got the article right. Caggiu means rennet or curd the substance that is used to transform milk into cheese, cheese translates as casu as in casu marzu (that could be translated as spoiled cheese).

[peoplefromibiza → C-x_C-f]

no, it's the Sardinian dialect for the Italian word caglio (rennet)