childish
immature / juvenile / simplistic
Adjective
comp: more childishsup: most childish
This term carries a strong negative connotation when describing adult behavior, implying a lack of emotional maturity, impulsiveness, or an inappropriate level of silliness. It suggests a failure to grow up or a regression into juvenile patterns of behavior. When applied to art or appearance, the word is more descriptive and neutral, referring to the simplistic qualities, naive perspective, or unrefined technique associated with a child's work.
Meanings
Adjective
behaving in a way that is typical of a child, especially in a way that is silly or immature.
"He is a 40 year old man, but he can be incredibly childish when he does not get his way."
Adjective
having the characteristics or appearance of a child.
"The painting was executed in a simple, childish style."