낙오돼요 · 낙오됐어요 · 낙오될 거예요 · 낙오된 (+noun)
to fall behind, to drop out, to be left behind
Example
산행 중에 한 명이 일행에서 낙오됐어요.
One person fell behind the group during the hike.
Word parts
낙 fall+오 ranks
Literally 'fall from the ranks' — dropping out of a marching line.