mesail
The visor of a helmet, especially when made in two parts.
historical
The mesail face-defence, still consisting of the visor with its eye-slits and noseguard attached to the helmet by two pivots, could be raised or lowered at will, while the two shell-shaped cheek-pieces formed a bevor or chin [guard].
Where it comes from
Probably from Old French muçaille (“concealment”), via French mézail; only attested in English since the 1800s (also spelled mezail, mursail). Alternatively, a connection to muzzle has also been suggested.
Adapted from the Wiktionary entry mesail (CC BY-SA 4.0).