To empty one's anus of fluid, usually semen but occasionally rotten fruit or liquified kabanas and kranskies, using a simple straw or straw like apparatus through which liquid is carried away. Thus preventing unwanted anal bacteria.
When one withdraws their penis from the anus too late during anal intercourse, and ejaculates a load of manly-ness into shit town, then a straw is used to suck up any unwanted semen that may cause anal infestations. "Shit, I pulled out too late and came in your asshole. But that's cool, we'll just have to give you drainal. I'll get the straw."
Jack had to perform drainal when a broken kabana was withdrawn from his best friends asshole.