This phrase is similar to "already know (something)" or "have known (something) for a long time". It's used when one is too lazy to speak the proper phrase. "Been knowed" seems to have a more powerful meaning though. Like you've known something so long that it's ridiculous that the other person trying to inform you of the topic that it is utterly ridiculous.

v. collectively or individually finding someone/something to blame for a problem, as opposed to brainstorming which connotes finding solutions to a problem

n. the act of finding someone/something to blame for a problem

(from a TV ad for HD television)