"large sea-shell," originally of bivalves, early 15c., from Latin concha "shellfish, mollusk," from Greek konkhe "mussel, cockle," also metaphoric of shell-like objects ("hollow of the ear; knee-cap; brain-pan; case round a seal; knob of a shield," etc.), from PIE root *konkho- (source also of Sanskrit sankha- "mussel") or else from a Pre-Greek word.
Since 18c. used of large gastropods. As a name for natives of Florida Keys (originally especially poor whites) it is attested from at least 1833, from their use of the flesh of the conch as food; the preferred pronunciation there ("kongk") preserves the classical one. Related: Conchate; conchiform; conchoidal.
The white Americans form a comparatively small proportion of the population of Key West, the remainder being Bahama negroes, Cuban refugees, and white natives of the Bahamas and their descendants, classified here under the general title of Conchs. [Circular No. 8, U.S. War Dept., May 1, 1875]