Jejune
Part of speech: adjective
Definition: Not nutritious. Lacking matter; empty; devoid of substance. Naive; simplistic.
Immature
Part of speech: adjective
Definition: Not fully formed or developed, unripe.Childish in behavior.
Example sentence: Everyone knows that life isn't fair. Saying it's not fair suggests that you think life is supposed to be fair, which makes you look immature and naive.