An elderly (or sometimes middle-aged) Korean woman who wears a large visor to shade her face from the sun's harmful rays. Some of the visors are absolutely massive, so much so that they often become a veritable black mask - the wearer's actual face becomes completely hidden from view. Plus, older women have a fearsome reputation in Korean society... hence the term "Darth Visor" (a pun of "Darth Vader", the lead antagonist from Star Wars).
Person 1: "Yikes, check out that old ajumma wearing a huge visor."
Person 2: "It's... Darth Visor."