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.
Dina: The new Snoop Dogg video is on the countdown.
Chris: I been knowed that fool.a
The Knowledge is the in-depth study of London street routes and places of interest that taxicab-drivers must complete to obtain a licence to operate a black cab. It was instigated in 1851, and has changed little since.
It is the world's most demanding training course for taxicab-drivers; and applicants will usually need at least 12 'Appearances' (attempts at the final test), after preparation averaging 34 months, to pass the examination.
Your mate doesn't stand a chance to pass the Knowledge no matter how hard he tries!