Dictionary of American Regional English
Cassidy, Frederic C., and Joan Houston Hall, eds. Dictionary of American Regional English. Six vols. Cambridge, Mass.: Belknap Press, 1985-2013. Volume 1: Introduction and A-C, Volume 2: D-H, Volume 3: I-O, Volume 4: P-Sk, Volume 5: Sl-Z, Volume 6: Maps and Supplementary Material.
The online version is here (paywall).
One of the most ambitious linguistic projects of recent years, DARE is an attempt to capture the regional slang and dialect of America. An unparalleled resource.
Copyright 1997-2016, by David Wilton