The acid-tongued comic mocked the Academy Award winning singer during an appearance on 'The Late Show With David Letterman' in February, when she recalled meeting the singer at the Oscars.
She said: "She was very scared because she was singing at the awards. She kept saying, 'My throat, my throat -- I don't know if I can swallow,'" Rivers said on the talk show. "And I said, 'Oh, yeah! You can swallow."
Now, Joan has refused to back down over her comments when asked to defend them during a chat with HuffPost Live, saying that the 'chubby' singer needs to lose weight.
"It's not mean. She's a chubby lady who's very, very rich, and she should just calm down or lose weight!" she said.
She wanted an apology, so I took an ad out on her ass. I said, 'You are not fat.' And then I had room for a lot of other ads," she quipped.
"Adele is beautiful and successful and has what, $100 million? Let's face reality: she's fat!"
It isn't the first time Joan has attacked fellow stars over their weight. She has famously criticised Christina Aguilera and Oprah Winfrey over their figures before.