what's eating you

Also, what's bugging you. What is annoying or bothering you? For example, We've conceded just about every point, so what's eating you now? or You're in a terrible mood—what's bugging you? The first slangy term, dating from the late 1800s, presumably uses eat in the sense of “consume”; the colloquial variant, from about 1940, uses bug in the sense of “annoy.” Also see what's with.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.