Bow Wave Gas
Flatulence preceding defecation; the natural bodily gaseous emissions that occur prior to a bowel movement, typically potent in aroma. Also known as pre-crap gas. Contrast normal farts that occur to eating such things as cabbage and refried beans.
"I'd been eating a lot of vegetables lately and had to hurry to the bathroom as my bow wave gas was really coming on strong, I knew I'd be prairie dogging next."