Noah Webster
Immature
IMMATU'RE, adjective [Latin immaturus; in and maturus.]
1. Not mature or ripe; unripe; that has not arrived to a perfect state; applied to fruit.
2. Not perfect; not brought to a complete state; as immature plans or counsels.
3. Hasty; too early; that comes before the natural time.
[In this sense, premature is generally used.]