childish

An adult being childish means they're acting like a little kid, often doing silly things or making immature decisions.

When people use the word "childish," it usually means that someone's behavior is not very grown-up. This can happen when an adult loses their temper and starts yelling at others, just like a kid might do. It could also be used to describe something someone did, like buying a giant stuffed animal or wearing silly clothes, because these actions are often associated with kids. For example, if you see your friend acting out in public, making tantrums and being very unreasonable, you might call their behavior childish. But keep in mind that the word "childish" can also be used as an adjective to describe a person who is acting silly on purpose, like someone doing a prank or playing a joke.