Polka Dot n.
When the Polka became the hottest dance craze in America manufacturers tacked Polka onto the names of numerous products in an effort to capitalize on the fad. (circa 1880).
Polka Dot seems to be the only surviving term from this less than brilliant marketing trend. I'd guess historically it also meant contrasting colored dots on solid colored fabric, but I have no idea if it was a brand name or simply a generic term.