@SeaTurtle
The Japanese word for elbow is hiji. The Japanese word for knee is hiza.
By having someone say "Pizza" ten times fast and then immediately asking them what the word for elbow is (by pointing to your elbow and asking what it is or something), the hope is that they will accidentally say hi
za rather than hi
ji, because saying p
izza is supposed to through them off.
In turn, the joke is that it doesn't work on Alice, who turns it around on the Duke by asking what the word for breasts is, it doesn't work on Rob because he's going senile and doesn't realise he didn't actually say pizza ten times, and Caph screws it up because she's dumb and doesn't know that the word for elbow is hiji, rather than hiza.
EDIT: Obviously, this works better when not translated, because they wouldn't be specifying that they want the
Japanese word for something, they'd just ask "What is this" or something similar.
In fairness to the translator, doing it this way is far better than how another translator for a different series did it. That translator decided to also translated the word elbow in the dialogue, which makes the wordplay the joke is based around meaningless.