Green’s Dictionary of Slang

Sooner n.

[SE sooner than; used of those who attempted to take over a territory before official permission, the main example was in Oklahoma, where eager settlers ‘invaded’ the territory before its legal opening on 2 Mar. 1889]

(US) an Oklahoman; also attrib.

[US]Wichita Eagle (KS) 13 Dec. 4/2: The question as to whether ‘Sooners’ could hold town lots has been mooted [...] the president’s proclamation ‘sitting down’ upon ‘sooners’ has fanned into a flame the smouldering embers and the ‘sooners’ are trembling in ther boots.
[US]Witchita Dly Eagle (KS) 21 Aug. 1/4: The class of people they call ‘sooners’ in Oklahoma [...] believe that if a day is set they can enter Oklahoma and secure lands [...] just as they did in 1889.
Pullman Herald (Wash., WA) 23 Nov. 2/2: There was more rumors of soldiers coming to put off ‘Sooners’.
[US]A. Adams ‘In the Hands of His Friends’ in Cattle Brands [Internet] Soldiers had gathered them into camps along the line to prevent ‘sooners’ from entering [Oklahoma] before the appointed time.
[US]W. Winchell On Broadway 3 Dec. [synd. col.] Things I Never Knew [...] Why Oklahoma is known as the ‘Sooner State’.
[US]Daily Ardmoreite (OK) 11 July 21/5: Sooners have less reason to be offended at being called ‘Okies’ than residents of other states have for their nicknames [DA].
[US](con. c.1900) J. Thompson King Blood (1989) 74: A sooner was a person who slipped across the border ahead of the starter’s gun. In years to come, it was to become an affectionate second-name for Oklahoma.
[US](con. 1949) J.G. Dunne True Confessions (1979) 330: Gang, she’s going to get married and have six kids. All those little sooners.