Sooner in the sense 'one who acts prematurely', i.e. a person who tried to get into the frontier territory of Oklahoma before the US government opened it to settlers in 1889.
Definition of Sooner State in US English:
Sooner State
proper nounˈso͞onər ˌstāt
informal
a name for Oklahoma
Origin
Sooner in the sense ‘one who acts prematurely’, i.e. a person who tried to get into the frontier territory of Oklahoma before the US government opened it to settlers in 1889.