A: You were talking on the phone for half an hour. Who was it?

B: It’s Mary. I feel awful for her — she’s been eating her heart out ever since she found out she was rejected by her top-choice school.

A: It’s not the end of the world. She can take a gap year and apply again. Now, cheer up and look at how well I dance now.

B: Wow, you dance really well!

A: Gene Kelly, eat your heart out!

Note: This hyperbolic expression from the 16th century alludes to strong feelings gnawing at one’s heart. It can either mean “to feel great sadness” or “to be very jealous.” In this second usage, the phrase is often said as an imperative and sometimes mentions a famous person (when the speaker jokingly claims to be more talented than that person).