Green’s Dictionary of Slang

1. (W.I.) a native of the Bahamas.

1852		N.Y. Weekly Trib. 1 May 7/3: Nearly one half of all residents [of Key West] are natives of the Bahama Islands. They are called Conch-men or Conchs, by reason of their skill in diving [DA].
1983		Maledicta VII 22: Conch, from the name of the edible gastropod, was a 19th-century name for Bahamians.

2. (US) a poor white native of the Florida Keys.

1948		Mencken Amer. Lang. Supplement II 137: The dialect of the Conchs, as they are called, who inhabit the Florida Keys, has been reported on by Thomas R. Reid, Jr [DA].
1983		Maledicta VII 22: By the 1920s conch was applied to West Indians in the U.S. Today, the nickname is best known as the proud moniker for a native of Key West, Florida – mostly whites.

3. a native of North Carolina.

1870		J.S. Bradford in Lippincott’s Mag. (Phila.) Nov. VI 458: The North Carolina conch is unquestionably the lowest specimen of the race known. He has absolutely no virtues, and is dirtier, if possible, than the negro. But he is not so lazy [DA].
1872		Schele De Vere Americanisms 45: He [i.e. a very poor person] appears as Conch or Low Downer in North Carolina, and as Sandhiller or Poor White Trash in South Carolina and Georgia.
1965		A. Green in Journal of Amer. Folklore 🌐 At one time or another Southern local colorists used these analogs for poor white: lubber, peckerwood, cracker, conch, sandhiller, redneck, cajun, woolhat, squatter, clayeater, sharecropper, linthead, swamprat, tarheel, hillbilly.