plain woman; unattractive woman
A somewhat blunt or old-fashioned noun for a woman who is not considered beautiful. Often used in contrast with 美人. Can sound direct or insensitive in modern conversation.
She thinks of herself as a plain woman, but in reality she is very charming.
In old novels, you often see phrases like 'a plain but good-natured girl'.
Formed from the prefix 不 (fu-, 'not') and 美人 (bijin, 'beautiful woman'). The prefix 不 is a common negative prefix borrowed from Chinese.