It comes from the old middle eastern concept of the scapegoat. You know the story, every year a goat is sacrified to carry away the sins of the village, one way was to put the goat in a sack and everybody in town spit in the bag, it is tied off and goat and sack are thrown into the river to float away, carrying the year's accumulated sins of the village with it.

In the unyielding eye-for-an-eye world of ancient Judaism, somebody has to pay for your sins, even if you didn't originally (yes, pun intended) commit them yourself. You still inherited them from Adam and Eve. Well, mostly Eve. The debt must be paid, with interest, even if you never borrowed the principle. You will recall, those were the days when your posterity was obligated to pay off your debts.

Somewhere, God is up in his throne, looking down at us foolish mortals, shaking his head, stroking his beard and muttering to himself, "Jesus Christ, how did these guys come up with this shit?"