A usually-derogatory term for people from "up north". This refers to the Union soldiers during the American Civil War who wore dark blue jackets and frock coats, which was actually the standard color of the Regular US Army for the past several decades leading up to War of Rebellion. (See also Yankees).
White Trash person on the porch in northern Alabama: "Hey, do y'all see thuh blue coats cummin' in thur Leen-cun towncawr down from Ohah-yuh? (Ohio)"