"Eat one's heart out" is an idiomatic expression that refers to feeling intense jealousy or envy towards someone or something. There are several synonyms for this phrase, including "green with envy," "jealous as a cat," "spitting mad," and "seething with jealousy." Another synonym for "eat one's heart out" is "gnaw one's envy," which suggests a persistent and nagging feeling of envy. "Covet" and "envy" are also related words that can be used to convey the same emotion. Overall, these synonyms allow writers and speakers to express envy and jealousy in a variety of ways, adding depth and nuance to their language.