To smack, slap or impede in any way the act of a sexual groping, especially from an annoying boyfriend who just wants to grab a titty or a cheek for grabbin's sake (because that's his ass to grab). This definition is reversible for male-to-female, except most men will opt out of the knock away and allow the grope in almost any instance.
Male: why do you keep blocking me?
Female: I didn't block you, I did the knock away.
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.