Note to Hamlet, 4.5.118-121: "That drop of blood that's calm proclaims me bastard, / Cries cuckold to my father, brands the harlot / Even here, between the chaste unsmirched brows / Of my true mother"

The general idea of this speech is clear: The King has just told Laertes to calm down and Laertes replies that if he were calm it would prove him to be a bastard, the son of a cuckold and a whore, because only such a bastard could be calm about his father's death. Thus, when Laertes refers to the "the chaste unsmirched brows / Of my true mother" he is asserting that his clean brows (symbolizing clean morality) show that his mother was no whore and he is no bastard. The idea that the brow reveals the truth of a person's character shows up in the proverbial expression, "honest as the skin between his brows," which Shakespeare uses in Much Ado About Nothing (3.5.12).