a) what a dog does with anything it can't eat, chew, or play with
b ) (A variation of "piss on it and walk away"): walk away from any experience, even a negative one, with your head held high, and move onto bigger and better things, instead of letting the experience frustrate you.
In terms of anger management adapted to modern society, especially under definition b) mentioned above, spit on it and walk away is more than a solid good piece of advice; the same can be applied to, to some extent, in dealing with negative people, or people you can't/don't want to get along with.
Drinking the plain away is your old school mid-week drinkingsession. It's when you're drinking just for the simple reason of having some fun in the midst of the plain boring existence that is your life..
-wanna go out for a beer?
-but it's Tuesday...
-c'mon dude, let's drink the plain away! 😄