Definition
question to a person who seems emotionally or mentally consumed by some
internal distress
Examples
1) What's eating you? You seem really upset today.
2) I can tell something is bothering you. What's eating you?
Etymology
It is likely from the idea of parasites eating away at food.
Synonyms
What's bothering you?, What's got you down?
- englishdaily626.com [edited]
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Maybe it was to answer that question, Dorothy Parker wrote
The sun's gone dim, and
The moon's turned black;
For I loved him, and
He didn't love back.