noun-modifying (irregular)
precocious; unchildlike; early-blooming
Describes a child who acts, speaks, or thinks in a way that seems unusually mature for their age. Often carries a slightly negative or critical tone, implying the child is not acting their age.
That kid says things that are so grown-up.
She's only 10, but she has a very mature way of thinking.
Formed from 大人 and the suffix じみる, “to take on the appearance or character of.” 大人じみた is its past attributive form, literally “having taken on adult-like qualities.”