'plainsman' originates from English, a compound of 'plain' + 'man'; 'plain' ultimately comes from Latin 'planus' meaning 'flat' or 'level'.

'plain' passed into Middle English from Old French 'plain', itself from Latin 'planus'; the compound 'plainsman' arose in American English in the 19th century to denote inhabitants of the Great Plains.

Initially it simply meant 'a man of the plain', but over time it came to specifically denote a resident or frontier inhabitant of the Great Plains (often implying a cowboy, settler, or rancher).