In Act III Scene III, Antonio pleads for mercy, but Shylock is deaf to his pleas. He declares that he will have his due from the bankrupt Antonio, no matter what Antonio says.
Shylock claims that the duke will grant him justice, and since Antonio has called him a dog before he has had reason to, Shylock advises him to be careful about his fangs. This just means that Shylock wants to exact retribution against Antonio for previous insults: if he is going to be known as a dog, he will definitely make his fangs felt.
I’ll have my bond. Speak not against my bond.I have sworn an oath that I will have my bond.Thou calledst me dog before thou hadst a cause.But since I am a dog, beware my fangs.The duke shall grant me justice.
let the forfeitBe nominated for an equal poundOf your fair flesh, to be cut off and takenIn what part of your body pleaseth me.