According to Wikipedia, there are multiple etymological theories behind the term armscye, aka the arm hole of a garment, but the usage of "arm scythe" or "armscythe" is incorrect.
I tend to get irrationally worked up over grammar, spelling, and semantics, so in my opinion it's a f**king disgrace. I probably won't say anything if I hear someone use it wrong because my mama raised me to be polite, and I generally try not to be a pedantic asshole, but sometimes my inner pedantry gets the better of me and I just want to scream "YOU ARE SAYING IT WRONG STOP IT STOP IT RIGHT NOW!" Like right now. So, "arm scythe" is out, the correct form, armscye, is in, and now you know.
I don't have any good pictures to go with this so here's a cute photo of my dog.