A Tennessee Alka-Seltzer is when you stick an alka-seltzer into your mouth, then have oral sex with a Southern girl(with out telling her, but of course). As her vagina gets wet, you slip in the alka-seltzer, stand back, and watch the 'fire works'...or, in this case, 'foam works'.
The 'Tennesse' part is derived from college kids out of D.C. going into Virginia on a Saturday night, and performing it on innocent country 'gurls'. Why did I not choose 'Virginia' to make it a 'Virginia Alka-Seltzer'? Because Tennessee sounds better. And if you don't like it, piss off!
Dude 1: Dude, I did a Tennessee Alka-Seltzer on a GRITS yesterday!
Dude 2: Shite! What happened? Was it fun?
Dude 1: Well...let's just say she'll never walk again.