"Dirtbike" refers to the penis of a male who has an unhealthy penchant for anal sex with his female partner. The name stems from the man's desire to ride the female's 'Dirt-Track'. Dirtbikes may also come equipped with "mud-tires" if the female is experiancing diarrhea at the time of intercourse, and "snow-tires" which refers to the male riding the 'dirt-track' after he has previously climaxed in it.
One must exercise caution when using a dirtbike on a fresh track however. The bike must be eased in to ensure that the fresh terrain could be ridden comfortably, avoiding any damage to either the bike and track. The process of "greasing the tires" with some sort of lubricant is probably a smart move for first time riders, and should be seriously considered.
"Adam loved riding his dirtbike in Carrie's dirt-track!"
"Adam loves anal so much that his dirtbike even comes with mud-tires!"
"Adam rode his dirtbike so hard that he got a flat tire!"