"lands of the American Midwest lying from roughly to the 104th meridian to the eastern slopes of the Rockies," 1755 (in singular form from 1680s), see plain (n.). Plains Indian is attested from 1844.
This region has a gradual slope from the mountains to the Missouri and Mississippi rivers, but is nowhere broken by any conspicuous ranges of hills. It is a region of small precipitation, wooded only along the banks of the streams, and not always there. The Plains and the prairies are not properly the same, from either a geographical or a climatological point of view. [Century Dictionary]