and thinks the movie is SUPER COOL
Flat What
"What."
A Stock Phrase when it comes to reactions, but a simple one. Although it is just one word, the way in which it is pronounced can speak volumes about how absurd (or mundane) the situation can be, simply by how and where the inflection in a speaker's voice (if any) is used. In written speech, it's supplemented by the punctuation following it. For example:
"What?" Standard reaction, used in everyday speech; slightly rising inflection (clarify what someone said, being asked a favor, etc.)
"What?!" Sharper inflection indicating a level of surprise (disturbing news, something you may need to sit down for, etc.)
"What!?" An accusatory or intimidating statement; "I'm in your face, what are you going to do about it!?"
"WHAT?!" "WHAAAAAT?!?!?!" et al; New levels of absurdity have been hit (Variations on the length of the word and (ab)use of ? and ! as needed; this is the one most often used for comic effect)
None of the above are examples of this trope. That's below.
"What." You've just been witness to something so bizarre, so eye-crossingly stupid (or possibly so mind-crushingly terrifying as to put you beyond the capacity for rational thought), that your brain no longer has the cognitive faculties to put together a more articulate response.
Think of it this way. You were trying to say WTF with the emphasis on F, but your brain short-circutied before getting to The.
And that's exactly how it's written, too: "what" with a period at the end. Some people cut this down even farther, going with "wha", "wat", "wut?" or similar. Sometimes appears on forums and image boards as the memes "what is this i don't even" and "i am 12 and what is this" (no punctuation whatsoever and the i not capitalized) and the immortal LOL WUT (all caps, spelling of the second word, and no punctuation whatsoever)— more complex, but still indicating that the speaker is too gobsmacked to be capable of a coherent sentence.
IMPORTANT TO NOTE: "What", in this context, is an interjection. Sentences consisting only of an interjection require an exclamation point. Interjections need to end with either an exclamation point or a comma (a dash or ellipse would also work), and (obviously) you can't end a sentence with a comma. "What!" is grammatical, as is "What?" (an interrogative), but "What." is not. However, "What." is how this trope is written, and "What." is how it shall remain. Just because it's not grammatical, doesn't mean it's not expressive. Dialogue is not required to be grammatically correct.