What Does "namchin" (남친) Mean in Korean?

This is a very common and casual term for 'boyfriend' among young people and adults. While 남자친구 is the full, proper term, 남친 is almost always used in everyday spoken Korean and informal writing (like texts). It implies a certain level of familiarity and closeness, so you wouldn't typically use it when talking about someone's boyfriend to a very formal elder or stranger, where the full 남자친구 might be slightly more appropriate.