It depends on the context, but unless you are talking about a literal slice of cake, it’s disrespectful to say to a woman because it refers to her body. Basically what it means is that you think she has a big behind or something along those lines. It’s not recommended to say this to anyone, especially not in formal situations, but even in casual situations it comes across as inappropriate.

If that's slang, I've never heard it - but maybe I'm too old. It's not offensive if he's really talking about cake.

Because of the word cake... It is a slang word for something else.🍑 I don't want to get banned from Italki so I will end there. 😅

While watching an episode

No. It's we were eating cereal