In a childish manner; in a way that shows immaturity or the qualities typical of a child.
From 'childish' + '-ly' (adverb suffix). The word builds on 'childish' which comes from 'child' + '-ish' (resembling or having qualities of), creating an adverb of manner.
Calling something 'childish' is interesting linguistically because we use the same word to mean both 'appropriate for children' and 'inappropriately immature for an adult'—the context completely flips the judgment from positive to negative.
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