Where did Shakespeare get the idea for a pound of flesh as forfeiture in the Merchant of Venice?
As with most cases in which an attempt is made to ascertain what a person writing centuries earlier "had in mind", there is no universal agreement on this topic. But there are some ideas that are worth considering.
1. There was much anti-Jewish sentiment in England in Shakespeare's day which led to many misunderstandings of who the Jews were and what they did. Since the bloody rite of circumcision was such an integral part of their identity, this gave them a reputation as a bloody people. With Shylock, the one who wanted the "pound of flesh" in the Merchant of Venice, he fell headlong into this stereotype. There were (false) rumors that Jews would kidnap children and use their blood in the Jewish Passover ceremonies. This gives you an idea of the picture people had in their minds of Jews.
2. What "pound" did he want? Since, in the play, he demands to pick the pound for himself, this is a valid question. Again, there is no sure answer.
Some make this an allusion to the aforementioned Jewish practice of circumcision.
Others believe there are indications that his intended target was the heart, which would fit pretty well with the spirit of the play.
Regardless of his intention (spoiler alert), his plans are foiled.