자만심이 · 자만심을
conceit, arrogance, overconfidence
Example
자만심 때문에 준비를 소홀히 했다가 시험을 망쳤어요.
I got overconfident, neglected to prepare, and bombed the test.
Word parts
자 self+만 arrogant+심 heart
Literally 'self-arrogant heart'.
Good to know
자신감 is healthy confidence; 자만심 is overconfidence that leads you to slack off.