What does "what's eating you" mean?

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.