I would be sceptical of any site that defines anything as synonymous with "shirking away", since that phrase makes no sense in English. "Shirking" means avoiding a duty, and is unconnected with facial expressions.
"Grimacing" is all about facial expression, basically on the spectrum between disapproval/dislike/disgust/revulsion.
The thing shared by wincing and flinching is that they are basically pain responses. Almost the same but not quite. One nuance is that wincing is more about actual pain received (pain here subsumes the mental sort, people even wince at bad puns), whereas flinching is about avoidance of anticipated pain. In some contexts it is considered cowardly, so that "unflinching" becomes a word for courage and determination, sometimes ruthlessness, whereas if there is such a thing as "unwincing", it's less common and a lot less likely to be heard in e.g. patriotic or revolutionary rhodomontade.
You ever had someone touch your eyeball? You can let them do it without flinching? It's not easy.
There is no obligation to have any facial expression when flinching. The body part under threat is withdrawn, that's all. Put your hand on a hot-plate, and you'll surely flinch, you may wince as well if it burns you. You don't have to grimace.
(I see this overlaps on Josh, who wasn't there when I started to compose)
Stoic is a commonly accepted alternative spelling to stoical. "The adjective (stoic) is recorded from 1590s in the "repressing feelings" sense, c. 1600 in the philosophical sense". (Etymonline)
of or relating to the school of philosophy founded by Zeno, who taught that people should be free from passion, unmoved by joy or grief, and submit without complaint to unavoidable necessity.
(lowercase) stoical.
(Dictionary.com)
The following Google Books search appears to suggest that the adjective stoic is becoming more common than stoical.
Best Answer
Google N-gram Viewer suggests that unexplainable is a recent invention (at least in written form):