What’s eating him, etc?
Definition 1 of 1
Pro Tip 1/3Emotional Insight
The phrase often hints at an underlying, unspoken emotional issue. 
She's been quiet all day, but won't say why. I wonder what's eating her.
Pro Tip 2/3Informal Context
It's commonly used in casual, informal conversations rather than formal settings. 
During the lunch break, Joe asked me, 'What's eating Mike lately?'
Pro Tip 3/3Figurative Language
The phrase is figurative and doesn't involve actual consumption. 
My little brother asked me, 'What's eating him? Is it a monster?'