cracker :

mid-15c., "hard wafer," but the specific application to a thin, crisp biscuit is 1739. Cracker-barrel (adj.) "emblematic of down-home ways and views" is from 1877. Cracker, Southern U.S. derogatory term for "poor, white trash" (1766), is from mid-15c. crack "to boast" (e.g. not what it's cracked up to be), originally a Scottish word. Especially of Georgians by 1808, though often extended to residents of northern Florida.

I should explain to your Lordship what is meant by crackers; a name they have got from being great boasters; they are a lawless set of rascalls on the frontiers of Virginia, Maryland, the Carolinas and Georgia, who often change their places of abode. [1766, G. Cochrane]