열등해요 · 열등했어요 · 열등한 (+noun)
inferior
Example
그는 형보다 자기가 열등하다고 느꼈어요.
He felt he was inferior to his older brother.
Word parts
열 inferior+등 rank
Literally 'of a lower rank'.
Good to know
The noun 열등감 'inferiority complex' is far more common than the adjective itself.