Stands for "defecation condition" - a play off of the government acronym for "defense condition". Refers to the varying, and usually scaling, levels of severity relating to an impending bowel movement. The direction of the sliding scale varies from user to user, but typical usage labels Defcon 1 as the least urgent, and Defcon 5 as an imminent expulsion (the opposite of the defense department's usage).
Dude, I was at Defcon 5 and nearly shit my pants. Luckily we drove by a Carl's Jr. and I was able to drop a load.