: not fully developed or grown. : acting in a childish way : having or showing a lack of emotional maturity.

Does immature mean childish?

not mature, ripe, developed, perfected, etc. emotionally undeveloped; juvenile; childish.

What does it mean to call someone immature?

For example, “immature” might be the adjective of choice to describe someone who seems unaware that if you don't keep track of your money, you run out of money. Or it could describe someone who appears to not understand that if you stay out all night, you will perform badly at your big meeting the next day.