look up any word, like muddin:
 
1.
A word similar to childish, used to define someone who acts like a kid in most cases. However it does not have to mean immature but simply acting like someone of a young age, or showing something that relates to a kid.
Examples:

My girlfriend wanted me to go to the playground with her then go back to her place and eat cookies and milk to me that just seems kiddish.

The toy aisle at walmart is very kiddish
by Zyonryu December 05, 2009