Can't mail it in

A common idiomatic expression!

"Can't mail it in" is a phrase that means you can't simply do the minimum required or put in the bare minimum effort to complete a task or achieve a goal. It's often used to convey that you need to put in more effort, be more dedicated, or take more initiative to succeed.

The phrase is thought to have originated from the idea that if you were to mail something, you would need to put in some effort to prepare it, such as addressing the envelope, affixing a stamp, and dropping it in the mailbox. Similarly, in life, you can't just "mail it in" and expect to get good results without putting in the necessary work and effort.

For example, if someone is asked to complete a project, but they're only doing the minimum required and not putting in any extra effort, someone might say, "You can't just mail it in! You need to put in more work if you want to get a good grade."