Childish or immature; characteristic of a baby or appropriate only for very young children.
From baby plus the suffix -ish (meaning having qualities of). A straightforward adjectival formation meaning baby-like.
Calling something 'babyish' is one of the harshest insults you can give a kid—it works because childhood is supposed to be temporary, and being called immature means you haven't grown up yet, which is everyone's worst fear!
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