We had a few people propose some armours to @bikiniarmorbattledamage due to some.. oddly designed besagews that were somewhat oversized - not quite sexualizing but also not quite making sense either.
More commonly they’re left out of depictions of armour, creating complex suits of plate where the armpits are easily exposed largely due to an expectation that blows would be directed and the head and general torso area (probably because in fantasy worlds swords seem to cut through plate... making it questionable why one would bother with the armour in the first place)
However, in real battles, techniques, armours and weapons all evolved in response to each other. So once plate made it difficult to harm warriors by blows to the chest or head they started targeting areas that were harder to protect.
Strikes to the armpits were somewhat awkward but devastatingly effective. Certainly at least as viable as attacking through an eye slot though by no means easy against a trained warrior. (Content warning: Some ableist language in the video below)
The term is also sometimes applied to similar parts set to protect knees from attacks too, though how and whether they are implemented or not is more complicated (mostly relating to whether they intend to fight on foot or from horseback).
- wincenworks
(Example image taken from Wikipedia)