When to use gangster in Korean
Korean: 깡패[ kkang-pae ]

When to use it
The word "깡패" means a gangster or a violent thug. It is used for a person who threatens, hits, or scares other people, so it has a strong negative meaning. Sometimes people also use it jokingly for someone who looks very rough or scary, but it can sound rude.
Example sentences
저 영화에 깡패가 나와요.
A gangster appears in that movie.
학생들은 학교 앞 깡패를 무서워해요.
The students are afraid of the gangster in front of the school.
엄마는 밤길에서 깡패를 조심하라고 했어요.
Mom told me to watch out for gangsters at night.
친구가 그 남자를 보고 깡패 같다고 했어요.
My friend said that man looked like a gangster.
가게 앞에서 깡패가 소리쳤어요.
A gangster shouted in front of the store.
지하철에서 깡패를 보고 모두 놀랐어요.
Everyone was surprised to see a gangster on the subway.