"What the what?" is slang that expresses surprise and/or disbelief at news. It is the type of slang that will be used until teenagers invent a new phrase. That is, until they get tired of it and move on. And then the phrase will die a very welcome death. More established equivalents include: "Wow! Really?" and "How surprising, is that true?" All of these questions are rhetorical: they don't expect an answer. But language usage (especially usage of newly coined slang terms) is always changing, so some people might use "What the what?" as an actual question. Unless you want to sound like a teenager or appear to be really 'hip' or 'cool', you never need to use "What the what?". And many times, when English learners use such slang phrases, it sounds *silly* rather than productive. You should certainly not use it with people you do not know very well or with your boss.

It's difficult to answer this without context but yes, I'd say it's perfectly acceptable. Whether something is considered polite or not is very subjective but I wouldn't have a problem if someone said this to me :) I most often hear it when I start asking someone a question beginning with "What…?" If they mishear a word or two they'll typically quickly say "What the what?" So it has a similar meaning to the word "Pardon?" or "Could you repeat the question please?" Of course if you want to be super polite then I would use the latter two examples. I hope that helps!

I'd never heard of "what the what." OK, I've looked it up and I'll explain it AFTER I explain something else. There are a number of expressions of the form "What the ____ ?" where various words fill in the blank. They all express some degree of angry surprise. How angry it is, and how rude the expression is, depends on what word fills in the blank. 1) You call a pizzeria and ask them to deliver a cheese pizza. The doorbell rings. The delivery person hands you a pepperoni pizza. You say "What's this? I ordered cheese, not pepperoni." That isn't particularly polite, but it's not impolite, and it's not bad language. 2) You call a pizzeria and ask them to deliver a cheese pizza. The doorbell rings. The delivery person hands you three pepperoni pizzas. You say "What the heck is THIS?" (Angry). Or "What the hell is THIS?) (Angry and a slightly rude). "I didn't order these. You got the orders mixed up, this is someone else's order." 3) You don't make any phone calls. Unexpectedly, the doorbell rings. Four delivery people from a pizzeria start bringing in six dozen pizzas. You say "What the !!!*&*$!!! is this????? I didn't order these! Take them the !!!*&*$!!! out of here!" (I'm using !!!*&*$!!! to stand for some bad word, possibly the "F-bomb.") You are extremely angry what you are saying is rude. Now, the explanation. "What the what?" is a catchphrase, a running joke, used by a particular character in a situation comedy: Liz Lemon. Do a Google search on "liz lemon what the what". Don't use it. It isn't in general use. Apparently it is a joke on "bleeping" bad words on television, or substituting weak euphemisms for them.

It's casual and direct (and a little comical), so you wouldn't use it in polite company. You can use it with people who you know fairly well, and you know that casual forms are acceptable to them. The usual rule for using casual and slangy speech is: wait for the other person to use it first! :)

However, even though it is a catchphrase, it is perfectly polite. "What the what" isn't bad language. People may be puzzled if they don't know the "Liz Lemon" character. If they know the character, they may wonder why you are trying to imitate her." But it isn't bad language.