Green’s Dictionary of Slang

keep one’s hair on v.

also keep one’s skin on
[? the image of tearing out one’s hair when in a rage; or ? (though prob. a folk ety. at best) the need of members of US pioneering wagon trains to keep calm in the face of an Indian attack (were they to panic, they might well be scalped, thus losing their hair)]

to keep calm, to keep one’s temper; thus get one’s hair off, to lose one’s temper.

[UK]Leeds Times 28 Mar. 6/5: The gentleman scoffed at this display of peevishness [...] and besought him to ‘keep ’is ’air on’.
[UK] ‘’Arry on Niggers’ in Punch 15 Mar. 113/2: When muffs spout such bunkum as that, tain’t so easy to keep on yer hair.
[Aus]Bulletin (Sydney) 24 Jan. 6/2: Launcelot’s screed is some of the ‘softest’ copy we have struck. We really advise the little man to ‘keep his hair on.’.
[Aus]Truth (Sydney) 6 May. 1/3: So See took the trip in a different ship, / And the Governor kept his hair on.
[UK]C. Rook Hooligan Nights 9: The butcher couldn’t ’ardly keep ’is ’air on.
[Aus]‘Rolf Boldrewood’ In Bad Company 215: Oh, he wouldn’t, eh? And why did Bill get his hair off?
[NZ]Eve. Post (NZ) Supp. 27 Dec. 1/4: All right, young man, keep your hair on.
[UK](con. 1950s) Nicholson & Smith Spend, Spend, Spend (1978) 66: Oh don’t get thy hair off!
[UK]B. Robinson Peculiar Memories of Thomas Penman 99: ‘Only a joke,’ he said. ‘Yeah, very fucking funny.’ [...] ‘All right, keep your hair on.’ .
[Aus]G. Seal Lingo 87: Someone may turn up her toes (die), or hang on by his fingernails. You can keep your skin on or alternatively jump out of your skin, depending upon the circumstances.

In exclamations

keep your hair on! (also have your head on! keep your head on! ...hairs on! ...tits on! ...wool on!)

calm down! don’t lose (emotional) control!

[UK]F.M. Crawford Dr. Claudius (2001) 92: ‘Keep your hair on, my young friend,’ replied Mr. Barker jocosely .
B. Harte A Ship of ‘49 (1912) 281: ‘Keep yer hair on!’ remonstrated the old man with dark intelligence .
[Aus]Dead Bird (Sydney) 21 Sept. 4/2: ‘Keep your hair on,’ yelled the leader, ‘We’ll get a crowd around’.
[UK] ‘’Arry on Equality’ in Punch 22 Feb. 85/2: Poor Buggins [...] Gets hangry, he do, at my talk. I sez, keep on your hair, my good bloke.
[UK]Kipling ‘Stalky’ in Complete Stalky & Co. (1987) 14: ‘Well, keep your hair on,’ said Beetle.
[UK]Marvel 24 Nov. 502: Keep your hair on!
[Aus]L. Stone Jonah 61: Keep yer ’air on, Joe.
[Aus]M. Garahan Stiffs 95: Now, sonny, keep your hair on.
[UK]Auden & Isherwood Ascent of F6 I iii: shawcross: It just happens that I take climbing seriously. You don’t. gunn: All right. Keep your hair on.
[UK]Whizzbang Comics 75: Keep your hair on, cocky.
[UK]H. Brown Walk in Sun 48: Keep your head on.
[Aus]Cusack & James Come in Spinner (1960) 159: OK, Mum, keep your hair on. [Ibid.] 272: OK, OK, keep your wool on. None of us’ll know for sure what’s coming till things are wound up.
[UK]J. Osborne Epitaph for George Dillon Act I: I was only asking. Keep your hair on.
[Aus]A. Buzo Front Room Boys Scene i: All right Thomo, keep your hairs on.
[UK]‘P.B. Yuill’ Hazell Plays Solomon (1976) 57: ‘Keep your hair on,’ I shouted as I went down the stairs.
[WI]E. Lovelace Dragon Can’t Dance (1998) 93: ‘Have your head on,’ Philo said, suddenly quite grave.
[UK]A. Payne ‘Senior Citizen Caine’ Minder [TV script] 56: Alright, keep your hair on. It’s the freedom, it’s gone to me head.
[UK]K. Sampson Powder 247: ‘We blew out a fucking fanzine from Japan!’ James giggled. ‘Keep your hair on, mate . . .’.
[US]J. Stahl Plainclothes Naked (2002) 281: Metallica Girl snapped her fingers in his face. ‘You gonna stand there or you wanna do something?’ [...] ‘Okay! Keep your tits on.’.
[UK]J. Baker Shooting In The Dark (2002) 212: All right [...] Keep your hair on.
[Scot]T. Black Ringer [ebook] n.p.: Aye alright, keep your fucking hair on!