1879 as colloquial shortening of Metropolitan (n.) "member of the New York Metropolitan Base-Ball Club."
THE baseball season has opened, and along with the twittering of the birds, the budding of the trees, and the clattering of the truck, comes the news that the "Mets were beaten yesterday 17 to 5." It is an infallible sign of spring when the Mets are beaten 17 to 5, and we invariably put on our thinner clothing when we read that refreshing, though perennial news in the papers. [Life magazine, May 12, 1887]
Used variously to abbreviate other proper names beginning with Metropolitan, such as "Metropolitan Museum of Art" (N.Y.), by 1919; "Metropolitan Railway" (stock), by 1890; "Metropolitan Opera Company (N.Y.), by 1922. Related: Mets.