허장성세가 · 허장성세를
bluster, empty bravado, putting on a show of strength
Example
그의 자신감은 허장성세에 불과했어요.
His confidence was nothing but empty bluster.
Word parts
허 empty+장 stretch+성 voice+세 power
Literally 'puffing up emptiness, inflating one's voice and power.'
Good to know
Often paired with 부리다: 허장성세를 부리다 means to bluster or put on airs.