Green’s Dictionary of Slang

upstage adj.

(US) conceited, snobbish.

1901		‘Hugh McHugh’ Down the Line 56: Clara Jane had about decided that Bohemia was away up stage.
1919		Van Loan ‘Mister Conley’ Score by Innings (2004) 430: So long as he’s hitting .325 [...] he can be as upstage as he likes.
1923		D. Hammett ‘The Second-Story Angel’ Nightmare Town (2001) 221: Quite a few of the guys [...] had been knocking me — saying I was up-stage and so on.
1939		A.O. Philipp ‘Vaudeville’ in Mss. from the Federal Writers’ Project 🌐 ‘Up stage’ was used to indicate proud and aloof manners. For example: George Hoofer lands a few big time dates and assumes a superior and patronizing attitude towards his old small time comrades, who remark: ‘George is getting up stage lately.’ ‘Yeah, what has he got to be up stage about?’.
1945		(con. WWI) J.P. Marquand ‘Good Morning, Major!’ in Mason Fighting American (1945) 426: Don’t be so up-stagy just because you have one of those what-you-may-call-’ems on your shoulders.
1960		E. North Nobody Stops Me 66: She’s decent enough, even if she’s a bit up-stage.

In derivatives

upstageish (adv.)

arrogantly.

1928

J. Callahan Man’s Grim Justice 36: We saunter into the dining room [...] We act very upstageish.