In the final segment of Com'è umano lui, a biopic about Paolo Villaggio, Maura tells Paolo about a phone call from Rizzoli. Here is the exchange:
Ha chiamato a casa Rizzoli. -Chi, l'editore? -No, quello del tonno. -Ah, che voleva? -Paolo, ma è ovvio che ha chiamato l'editore. Ma perché ci dovrebbe chiamare quello del tonno, scusami? Tu sei un tonno.
Rizzoli called at home. -Who, the publisher? -No, the tuna one. -Ah, what did he want? -Paolo, but it's obvious that the publisher called. But why should the tuna one be calling us, excuse me? You are a tunafish.
Captions 63-69, Com'è umano lui Film - Part 30
Play Caption
Maura is answering Paolo's first question, "Chi, l'editore?" Rizzoli is the name of an important publishing company, perhaps, at one time, the best known of them. Most Italians have heard of Rizzoli Libri (Rizzoli books). Maura's answer is sarcastic because she thinks it should be obvious, so she says, No, quello del tonno (no, the tuna one). Why does she say this? Because Rizzoli is also a very well-known producer of canned fish. Particularly appreciated are their anchovies in olive oil, but Maura mentions their tuna. And then she calls him a tuna.
Readers might not have caught the connection, so here is a photo taken of a jar of tuna, bought at the supermarket.

Paolo clearly doesn't get the sarcasm at the beginning (or is pretending not to) and asks what they want. So she has to explain, she means the publishing house and then calls him a tuna.
Paolo Villaggio has written books (for Rizzoli) and appeared in movies. Below are some Wikipedia links about the actor and author.
Here is an article in Italian.
Here's the same article in English.
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