Immature means not fully grown, or not behaving like an adult. An immature bird, emotionally immature, immature behaviour.
In biology it is neutral and technical: an immature specimen has not reached its adult form. Of a person it is a criticism about behaviour rather than age, and it is a serious one when used of an adult. Note the pronunciation, im-uh-TURE or im-uh-CHURE; immaturity; and that mature and immature are used of cheese, wine and people, with the compliment reversing between them.
1adjectiveimmatureNot fully grown.
“The immature apples were small, green and too sour to eat.”
2adjectivebehavingNot behaving like an adult.
“Stop being so immature and apologise to your sister.”
immature is a state of not being fully grown.
Latin immaturus is in-, not, plus maturus, ripe or timely. Something immature has arrived too early rather than failed to grow, which is a slightly gentler judgement than the English word carries.
Proto-Indo-European
*meh₂-
Middle French
immature
English, today
immature
From Middle French immature. Partially displaced unripe, from Old English unrīpe (“unripe, immature”).
Etymology from Wiktionary, CC BY-SA
Practised in Word Building