To disregard, discredit, or disrespect.
Throwing dirt on someones name.
*Though used in Canada, it is not only Western Canadian slang. It is used commonly in the US ranging widely from NY to California since the late 70's (if not before that).
It is also a popular word in the U.K.
Me: What happened to your eye?
John: Mikey and I got into a fight over Cecilia and he snuffed me in the face.
Me: Wait, what? Cecilia that girl you've been dating?
Me: Wow and he was supposed to be your homeboy too... Damn you were dirted hardcore.
John: I know he shitted on me, but paybacks a bitch.