An assdick is a penis that eminates and protrudes from the ass. Those with an assdick were either born with it, or had it surgically grafted on by a man known only as Dr. Bob.
The purpose of the assdick is to block the entrace of unwanted objects (such as other penises) into the anal cavity. When the assdick senses an incoming threat, it immediately erects to disallow entry.
OMG that fag tried to buttfuck me, but luckily my trusty assdick was able to protect my anal virginity.