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.
Something you say when someone says something weird or embarassing that no one else experiences in a way that seems like it happends to everyone, making you beleive it happend to them