And then specifying "younger" and the gender of the cousin is straightforward enough.
As a commenter in the thread two years ago noted[1], this represents (in Agnatic cultures) “female cousin who is not part of my close family (different last names)”.
Someone else linked a page on Chinese kinship terms but it gave an incorrectly simplified definition for the same term so I didn't realize it was a match.
There is also a distinction between older and younger siblings, and then also gender.
What seems awkward in English is the very simple and concise 表妹 (biao mei) in Chinese.
We had a terribly hard time explaining the idea of "cousin." Once we did, she explained that she was unfamiliar with the concept because her family (and apparently all the families she knew well) had only had single children for generations.
I assumed there had to have been a word for it at some point, and that people were probably using it somewhere, but she seemed quite sincere in telling us she knew no such word.
I also discussed the indirect/cultural factor where one should learn about this. There are two complementary factors here: if the young woman was rural there was practically no way she never encountered a family with multiple children; if she was urban it was more likely she was educated and thus should at least learn about it from school and/or through literature.