1. What does it mean?
여사친 shortens 여자 사람 친구. It makes clear that a woman is a friend rather than a girlfriend, avoiding the romantic reading that 여자친구 usually has.
Also written or said as 여자 사람 친구, yeosachin
2. The nuance
Use it when that distinction actually matters. The label is not an invitation to question a friendship or speculate about someone's feelings. Plain 친구 remains the simpler choice when there is no possible misunderstanding.
3. Examples
“여행 일정은 서울에 사는 여사친이 추천해 줬어.”
A female friend who lives in Seoul suggested the travel itinerary.
“그 사람은 여자친구가 아니라 오래된 여사친이야.”
She isn't my girlfriend; she's a longtime female friend.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.