Derived from the expression "Billy Bad Ass". To Billback is to return in full and/or a plethora, any act of total sweetness and Bad Ass-ness which has results in your ultimate demise and/or embarrassment to the original Billster/Billmaster from which the initial Bill originated.
A Billback is simply the retaliation on a Bill, with a Bill.
In essence the only true way to recover from being Billed. A Billmaster has highly honed Billback abilities which set him a cut above a Billster.