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"immature" is also used as a crossword clue. If that's what you saw, the answer is often childish (8 letters).
Definitions
(of birds) not yet having developed feathers
Word origin
From Middle French immature. Partially displaced unripe, from Old English unrīpe (“unripe, immature”).
Used in a sentence
“And thou also canst best account for the causes of her immature death […].”
“You're only young once, but you can be immature the rest of your life.”
“The man was immature for throwing a tantrum.”
Source: Wiktionary, CC BY-SA 4.0
Used as a crossword answer2 curated clues
01“Not grown up”8 letters
02“Not fully grown”8 letters
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