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Webster 1913 Edition

adv.

In an immature manner.

Warburion.

adv.

Too soon; before ripeness or completion; before the natural time.

immaturely ‎(comparative more immaturely, superlative most immaturely)

Giving an appearance of immaturity.
They may be approaching 40, but after three beers they are behaving as immaturely as any teenager.
Without maturity.
He immaturely chose the sports car instead of the money.