Taken as an idiom from William Shakespeare's "The Merchant of Venice" in which the money-lender Shylock, in remuneration for a defaulted debt, demands a "pound of flesh" in compensation. The idea is first introduced in act 1, scene three about line 140 with Shylock speaking:

This kindness will I show. Go with me to a notary, seal me there your single bond; and, in a merry sport, if you repay me not on such a day, on such a place, such sum or sums as are express'd in the condition, let the forfeit be nominated for an equal pound of your fair flesh, to be cut off and taken in what part of your body pleaseth me.

Though in this context it is easy o assume that the character of Shylock is a vile and evil man, Shakespearean historians generally that condition is set as a gentlemanly jest between business friends and not intended as an actual condition... until later in the play when Shylocks' honor is violated by one of the protagonists eloping with his daughter.

Later, a4s1, in a Judges chambers, Shylock argues his case with a magistrate of Venice and through much deliberation it is decided that, even if the remuneration was set in friendly jest, the bond stands and Shylock is entitled to "a pound of flesh". However, this never happens because Shylocks daughter, Portia intervenes thusly(line 310):

Tarry a little; there is something else. This bond doth give thee here no jot of blood; The words expressly are 'a pound of flesh:' Take then thy bond, take thou thy pound of flesh; But, in the cutting it, if thou dost shed one drop of Christian blood, thy lands and goods are, by the laws of Venice, confiscate unto the state of Venice.

(line 330)
Therefore prepare thee to cut off the flesh. Shed thou no blood, nor cut thou less nor more but just a pound of flesh: if thou cut'st more or less than a just pound, be it but so much as makes it light or heavy in the substance, or the division of the twentieth part of one poor scruple, nay, if the scale do turn but in the estimation of a hair, thou diest and all thy goods are confiscate.

Basically what the point here is that "he who lives by the sword, dies by the sword". If Shylock was intent on the letter of the law, he too will be bound by it.

Shylock relents and withdraws his claim without the judgement, saying (line 343):

Give me my principal, and let me go.