Immature
/ˌɪməˈtjʊr/
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Definition
(adj) Occurring before the proper time; untimely, premature (especially of death).
Sentence Examples
My brother is an immature boy.
He was so immature, he couldn't resist the temptation.
CEFR Practice Quiz
The young puppy's ____ behavior was occasionally annoying, but he was always very cute.