"Keep your jaw on" is a phrase used when somebody is overwrought, overly shocked, or unnecessarily upset over a trivial matter. The expression was inspired by John Glubb, a WW1 soldier who had his jaw shot off but managed to have it reconstructed using surgery. The point of the phrase was, if even somebody who's reached a situation that low managed to fix their problems, you shouldn't be reacting so badly about a smaller issue.
Popularised in Northwest England's coastal towns, the phrase is often used as a less niche form of the phrase "don't get your knickers in a twist."
PERSON 1: “Oh my God, my life is over! My phone, my beautiful phone - it's been delayed in transit. I literally don't know what to do. What the Hell should I do?”
PERSON 2: “Jesus Christ, keep your jaw on. It'll clearly get here in the end - some people have real problems.”
A Jawon is very athletic. He is a little shy. He is a good child but he does like to fight a little. He is very smart and likes to learn. He like to draw and likes to learn about animals. His favorite dog is an Alaskan Husky. His favorite color is green. He loves football and hopes to go to the NFL one day in his future. He likes to workout and get better at football. He likes to run and get prepared for track. He is very goofy and like to make people laugh.