shirt over pants?!

[English4everyone]

Sometimes men wear formal clothes in a neat way. This way, I think, they have to put the shirt in the pants.. But sometimes, usually in informal cases, men don't put the shirt in the pants and they leave it over the pants.
How can I say the parts in bold in good and common English?

 

[JMurray]

In my part of the world we say that they tuck their shirt in or they don't tuck their shirt in.

not a teacher

 

[Barb_D]

And you might here that someone's shirt has come untucked.