So imagine you ask a person a serious question, and they make light of it.
You feel a certain emotion which I'm trying to find the name for. Your shoulders may slump. Face and body seem to relax. Eyes half-close in a kind of tired/dejected way, and you look at them pointedly, having expected something else but are not surprised. Perhaps you even expected to be disappointed in a small way. Lips may purse slightly.
Another situation where a person might react this way is if they observe someone else's behavior they dislike, but not to a great extent, and they're tiredly resigned to the other person behaving that way. So they just watch with the above-described expression on their face.
Other than being disappointed and mildly dejected, there might be a tiny bit of something like irritation without the anger, just resignation, but all of these emotions are too mild to be the right word for this reaction.
Despite the fairly significant bodily reaction (i.e. the facial expression, body relaxation, etc), the person recovers quickly with a "Darn you!" or "There he goes again" feeling, and could just go on with their day afterwards with hardly any lingering feeling at all.
There must be a word for this, but for the life of me I can't think of it!
EDIT: It has been brought to my attention that
single-word-request requires an example sentence, so without further ado, it might go something like this: "For the umpteenth time, when she asked him a serious question, he just laughed it off and changed the subject. She looked at him, ____________."
For a long time, I thought the right word here was "nonplussed," but I recently discovered it is not. It is not befuddlement or being unconcerned, though the physical expression may actually be quite similar to when nonplussed is used in a sentence.
Here's an example of something like it in the "Got Milk," "Did you think I wouldn't find out?" commercial:
https://www.youtube.com/watch?v=UnzK9l5MGCo (At 6 seconds in.)