If "girlfriend" is 여자 친구, how do you say "female friend" in Korean?

ANSWER: 여자 사람 친구, which is abbreviated to 여사친

In English, the word "girlfriend" implies a romantic relationship, especially if the relationship is between a man and a woman, but "female friend" does not. In Korean, 여자 친구 translates as "girlfriend," and 여자 사람 친구, which literally means "female person friend," translates as "female friend." Of course, 여자 사람 친구 is too long for the average Korean teenager, so they have abbreviated it to 여사친, formed using the first syllable of each word.

Likewise, "boyfriend" implies a romantic relationship, but "male friend" does not, so Koreans would say 남자 친구 if there was a romantic relationship and 남자 사람 친구, or just 남사친, if there was not.