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general::
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.
American Heritage Idioms