The Collaborative International Dictionary of English v.0.48:

Sooner \Soon"er\, n.
   In the western United States, one who settles on government
   land before it is legally open to settlement in order to gain
   the prior claim that the law gives to the first settler when
   the land is opened to settlement; hence, any one who does a
   thing prematurely or anticipates another in acting in order
   to gain an unfair advantage.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

sooner
    adv 1: comparatives of `soon' or `early'; "Come a little sooner,
           if you can"; "came earlier than I expected" [syn:
           sooner, earlier]
    2: more readily or willingly; "clean it well, preferably with
       warm water"; "I'd rather be in Philadelphia"; "I'd sooner die
       than give up" [syn: preferably, sooner, rather]
    n 1: a native or resident of Oklahoma [syn: Oklahoman,
         Sooner]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "sooner":
   before, beforehand, by choice, by election, by vote, colonial,
   colonist, colonizer, earlier, first, homesteader, immigrant,
   in preference, nester, pioneer, planter, precursor, preferably,
   rather, rather than, settler, sooner than, squatter