childish, puerile
Kindisch, unreif oder töricht in seiner Ausdrucksweise oder seinem Verhalten.
Exhibiting childish immaturity or silliness.
☞ Often encountered in the form 'tümelnd'.
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Sein tümel Verhalten war für sein Alter unpassend.— His childish behavior was inappropriate for his age.
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Sie warf ihm einen tümel Blick zu.— She cast a puerile glance at him.