Similar to white girl wasted, birthday baked means a middle class person or someone on a budget spending more on marijuana for their birthday as a treat to themselves.
Dave: What are you doing for your birthday?
Harry: I just got paid so I'll probably visit my dealer and get birthday baked.

John planned to get birthday baked and therefore had to buy more snacks than usual.
by CadillacDan July 20, 2011
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