남사친
namsachin
A guy friend — a male friend who's just a friend.
OriginShort for 남자 사람 친구 (male person friend), to separate it from 남자친구 (boyfriend).
When to useClarifying that someone isn't your boyfriend.
CarefulCasual — fine with friends, out of place in formal writing or with your boss.
Examples
걔는 그냥 남사친이야.
He's just a guy friend.