Green’s Dictionary of Slang

above oneself adj.

over-confident, pushy, esp. of someone who is usu. more self-effacing; often in phr. get above oneself

Kipling From Sea to Sea (1899) I 455: I have [...] seen more decent men above or below themselves with drink, than I care to think about.
[Aus]Bulletin (Sydney) 15 Nov. 14/3: Many otherwise quiet horses, if well-fed and ‘above themselves,’ will chase and attack sheep, calves, dogs, etc., biting, kicking, and striking-down with the forefeet.
[UK]A. Binstead Pitcher in Paradise 138: Both were feeling absolutely fit — a bit above themselves in fact.

In phrases

get above oneself (v.)

to act in an arrogant manner, to be self-satisfied.

[‘Vieille Moustache’ Barb and Bridle 98: Finally, that no lady should ever ride a horse of high breed and courage that has been allowed to ‘get above himself,’ by remaining day after day in the stable, or having insufficient work].
Reports of committees: 30th Congress, 1st session - 48th Congress, 2nd session 175: The nigger will get above himself, and above their business; and [...] you are in danger of losing your own life.
Army and Navy Magazine 38: Should you have a good inspector of gunnery, give him all the latitude you can [...] If you make too much of him he will probably get above himself; but that [...] is a fault on the right side.
[UK]Punch 23 Sept. 137: Lest the spirits of the travelling tourist should rise so high that he might run the chance of ‘getting a bit above hisself’, as horsedealers graphically express it.
[Aus]Bulletin (Sydney) 12 Sept. 2/4: The stewards invariably perform this act in the case of any lad who gets a bit above himself – which accounts for the circumstance that the British jock [...] is about the most humble, self-abasing, forelock-dragger on God’s green earth.
[UK]W.S. Maugham Bread-Winner Act I: You oughtn’t to let him get away with them. He only gets above himself.
[UK]R. Westerby Wide Boys Never Work (1938) 40: They were all the same, complacent, [...] scared of ‘getting above themselves,’ expecting nothing.
[Ire]G.A. Little Malachi Horan Remembers 67: What with her being in such demand about here, she got entirely above herself.
[UK]A. Buckeridge Jennings Goes To School 39: They were merely administering a mild rebuke [...] so that he might know his lowly station in life and not get above himself.
[Ire](con. 1930s) L. Redmond Emerald Square 137: Kevin O’Connell, a big thick [...] got above himself and warned me if I bumped into him again, I was ‘for it’.
[UK]Guardian 24 July 10: One Tory MP complained this week that she was ‘getting above herself’.
M. Millard-Brawn What My Mother Told me 14: Aunt Ethel thought she was better the others when she married Uncle John, ‘Who’se getten above hersen,’ Grandma Bolton would say according to what Mother told me, meaning ‘She is getting above herself, she thinks she is better than us’.