How popular is the phrase “What’s eating you?” Is it an interjection?

I saw an American English teacher introducing the phrase, “What’s eating you? in a TV English conversation class this morning by warning audience not to confuse it with the question asking what food you are eating.

As I was unfamiliar with this expression, I consulted with OALD. It registers this as an idiom used to ask what sb is annoyed or worried about.

Wikidictionary defines it an interjection meaning “what is annoying you?”

However, Googlebooks Ngram Viewer doesn’t register “What’s eating you?”

Is “What’s eating you?” a popular phrase both in Britain and United States?

Is it an idiom or interjection as defined by Wikidictionary in the same sense of “Oh my God,” “Look out,” and “Watch it”? To me it looks a plain interrogative sentence.

What is the origin of “What’s eating you?”

Solution 1:

It's a common (informal) turn of phrase, but I wouldn't call it an interjection. As OP suspects, it's an interrogative sentence. As Tolerance72's revised NGram link suggests, it's somewhat "dated" slang that peaked during the war years for Americans (a couple of decades later for Brits, who often play catchup with American slang).

Note that you can't "transform" it (into "What are you being eaten by?", or "This problem is eating me!", for example), so I guess that in itself makes it an "idiomatic" usage.

So far as "origin" is concerned, I doubt that such a trivial metaphoric usage could really be traced. Here's an instance from 1872 (I imagine there are earlier ones), but it didn't really start to gain traction until the inter-war years.

In the excellent 1993 movie What's Eating Gilbert Grape, Gilbert Grape (Johnny Depp) is trapped in a humdrum smalltown existence, his life dominated by caring for his mentally challenged brother Arnie (Leonardo DiCaprio at his very best), and morbidly obese mother.

In fact, Gilbert never actually complains (because he's too "nice"), and he gives few outward signs of being dissatisfied with his lot in life. But that's a slightly unusual context for the phrase - usually it's a question asked of someone who's obviously discontented (often conveying exasperation, rather than genuine concern).

Also note that there is a similar usage "Eat your heart out!", usually followed by someone's name (who may not actually be present to be directly addressed). That one is an exclamation, implying that the named person is/would be crestfallen on learning of something the speaker has just done or discovered (and which casts the named person in a bad light, or contradicts their stated position, for example). I couldn't say for sure whether they share a common origin, but I suspect not.

Solution 2:

I believe the question mark in your Ngram search interfered with your results.

See here for the answer you were seeking.

Interestingly, it appears the phrase is undergoing a resurgence.

Solution 3:

I have heard it several times, I think broadly represented in UK/US/AU, though I couldn't be 100% sure on that. Generally inquisitative more than interrogative, mostly spoken in a friendly tone, as you would say to a friend feeling down: "What is up with you?"

As to the origin:
When a person is feeling upset about something, but holding it in, the person may look like or even feel like he is being eaten from the inside. A friend noticing this would use the expression to cheer the person up.

Or in a second case when the person is feeling upset about something, but holding it in: At some point that person might burst out in anger or snap at people. Then a friend experiencing this unusual anger would use the expression (in a sharper tone) to calm the person down. Often meaning: "What have I done to deserve your anger?"