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Explanation: it is a heavy-duty insult aimed at the dead (morti) of the person being insulted. It is the pejorative of morti (morti + accio, in the plural); tua is Roman for tuoi (il romano e\' il mio dialetto); the denotative meaning, however, is no longer active, i.e. when I hear it I no longer take offense because my dead grandmother is called into question. So I would go, as Ilde suggests, with a heavy-duty insult of your choice, maybe quaint and regional, depending on the variety of English you are using.