첫째 아들 · 첫째 날 · 첫째로 (firstly)
first; eldest (child)
Example
우리 집 첫째는 올해 대학생이 됐어요.
The eldest in our family became a college student this year.
Word parts
From 첫 'first' plus -째 'order'; the ordinal for 하나.
Good to know
Ordinals add -째 to native numbers, but 'first' is irregular: 첫째, not 한째.
Related words