PULL YOUR SOCKS UP

Learn a new word every day..
Pull your socks up : to ask someone to behave properly or to ask someone to improve or perform better. You can also say , pull up your socks, if your aren't satisfied with someone, and want them to do better, or improve at some thing. 

Note one thing : You should only say this idiom to someone, such as your child, your student, your employee, or possibly to a friend. You shouldn't say it to someone like your boss, your teacher, your parents, or anyone you should speak to respectfully.