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.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.
FRIEND A: "Did you just take a stealthie of me?"
FRIEND B (turning phone around): "no I was just using snapchat's new filter, see?"