how sharper than a serpent's tooth quote

The quote "How sharper than a serpent's tooth it is to have a thankless child!" is from William Shakespeare's tragedy King Lear. It appears in Act 1, Scene 4, where King Lear expresses the painful sting of having an ungrateful or unthankful child, comparing that pain to the sharpness of a serpent's tooth, i.e., a snake bite. In the context of the play, Lear is lamenting the betrayal and ingratitude of his daughter Goneril after he has given her power.