The word ‘plain’ in its original sense just meant ‘flat, smooth’. In the thirteenth century, it also gained the meaning ‘evident’. And from the fourteenth century on, ‘plain’ has been used in the way we understand it in the term ‘plain language’: to refer to something simple, clear, and obvious without superfluous ornamentation: http://en.wiktionary.org/wiki/plain and http://www.etymonline.com/index.