is when one neglects to clean or wipe properly betwixt their glute cheeks and a flaky orange/brown build up of poop deposit happens. Butt rust has been known to emit a foul odor of human fecal matter and cause extreme and even painful discomfort when walking, running and especially biking. Butt rust is not only a human malady but is most common in and among other bipedal or quadrupedal mammals whose legs come together near the exterior of the anus to form a crevice or fissure of the body perfect for the formation of said ailment. Butt dust can often be embarrassing as it can appear in your undergarments as skid marks, it can leave behind butt dust (see butt dust definition), or it can cause grief and later chafing when one is expecting to expect butt sex. The only sure fire ways to rid oneself of butt rust is to clean between the buttocks thoroughly and regularly and to perform extra in depth wipes after leaving a deposit in the porcelain pot.
"After not bathing for a few days I was washing my tush in the shower when I noticed the water turned an orange/brown color which must have been from my butt rust."
"I was going down on Tammy the other day when I accidentally licked her anus and she had butt rust there!!"
"I didn't know what to expect when having butt sex but I knew I needed to take care of my butt rust before the main event."