Of Jamaican origin. The literal translation to the standard North American English dialect would be "you already know" or, "you know it!". Used (to the writers knowledge) mainly in Toronto, Canada (in its North American variation) where it has found use among the wider (mostly younger) population and, of course, in its original Jamaican patois dialect everywhere Jamaicans are found (that's globally!).
a) Emphasize the validity of a fact (usually stated immediately after the phrase) - e.g. "You done know they're running a scam."
b) Respond affirmatively to a query - e.g. Q: "Are you good for this money I'm lending you?" A: "You done know"
c) Express one's assent or agreement - e.g. A: "Boy, it's hot out today!", B: "You done know"
The phrase is also varied to fit the context of the conversation by changing the pronoun used (i.e. "She done know", "They done know", "I done know", etc.)
Jamaican dialect example:
"Yuh done know di gyal a lie 'bout 'ar panty size. Everybody know seh 'ar batty bigga dan dat!" (insert laugh here)