keep her hair on

Harry Potter and the Philosopher's Stoneを読んでいます。Harryは11歳の時にwizardの学校に行くことになり、ロンドン駅から汽車に乗ります。
“Hey, Mom, guess what? Guess who we just met on the train?”
Harry leaned back quickly so they couldn’t see him looking.
“You know that black-haired boy who was near us in the station? Know who he is?”
“Who?”
“Harry Potter!”
Harry heard the little girl’s voice.
“Oh, Mom, can I go on the train and see him, Mom, oh please…”
“You’ve already seen him, Ginny, and the poor boy isn’t something you goggle at in a zoo. Is he really, Fred? How do you know?”
“Asked him. Saw his scar. It’s really there--like lightning.”
“Poor dear--no wonder he was alone, I wondered. He was ever so polite when he asked how to get onto the platform.”
“Never mind that, do you think he remembers what You-Know-Who looks like?”
Their mother suddenly became very stern.
“I forbid you to ask him, Fred. No, don’t you dare. As though he needs reminding of that on his first day at school.”
“All right, keep your hair on.”
A whistle sounded.
子供向けの本なので慣用句は余り出てきませんが、"keep your hair on" は明らかに慣用句ですね。知らない慣用句なので辞書を見ます。
・Farlex Dictionary of Idioms: To remain patient and calm, especially when beginning to become angry or upset with impatience. Usually said as an imperative. Primarily heard in UK. Yeah, yeah, keep your hair on! I'll have the report finished by the end of the day! A: "That lady is pestering us about getting her car back today." B: "We need another day to get it fixed. Tell her to keep her hair on!"
・The Dictionary of American Slang: To stay unruffled; be calm; cool it: I'm coming. Just keep your hair on, won't you?
・Cambridge Idioms Dictionary: Keep your hair on! (British & Australian informal): a slightly impolite way of telling someone who is angry to try to be calm and patient Keep your shirt on! I'll be with you in a second.
"keep your shirt on" と同じ意味ですか、それなら "6/16/2015" に取り上げましたね。