Babyish

INFANTILE

Think of a word that describes behavior more suited to a very young child, often used to criticize immaturity in adults.

"Babyish" directly hints at something childlike, and "infantile" is a formal word meaning exactly that—behaving or relating to an infant or young child. The clue plays it straight with no hidden wordplay or tricks, relying on the simple definition. If you think of "infant" (a very young child) and add "-ile" (a suffix meaning "relating to"), the answer clicks into place. It’s a straightforward synonym match.