Flat tongue is used to refer to the American - USA (excluding persons with Georgia, Texas, and New York accents) accent.
Speak with an educated person from the United States, excluding persons from Texas, Georgia, and New York with accents, and you'll hear the flat tongue accent.
The act of pressing ones tongue against the lower backbone of your woman, then drawing, with your tongue of course, a wet, sloppy path to her bellybutton. Also known as the Backbone to Bellybutton kiss.
Dood, that chick is soo hot, I would flat tongue her after she ran a marathon, stink and all.