Meaning of “Pizza(ピザ)”

“Pizza(ピザ)” means fat in slang.
This is a fairly old slang term, and it likely originated from the stereotype that overweight people eat a lot of pizza.
This expression is basically an insult.

Example Conversation

A: That’s exactly why you can’t get a girlfriend.
B: Shut up! You’re the last person I want to hear that from, pizza!

A: お前そんなんだから彼女出来ないんだよ
B: うるせー、ピザに言われたくねー