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http://www.medeiros.org/history.html

also:

[...Sometimes the correct spelling of a Surname can be determined by locating the place of origin for your original ancestor into this country. I believe the original spelling of the surname is Medeiros, Madeiras or Medearis. These spellings are all Portuguese meaning "One who came from Medeiros"....]

the above text is taken from the following website:
http://www.mindspring.com/~kellcin/history.htm

what M.S. says is highly unlikely, but there is a surname Metayer. that is probably what he meant.