This famous quote from the play comes in Act I scene 4 just after Lear has been insulted by Goneril, who has asked him to reduce the numbers of people in his train and to make sure that they are of a similar age to Lear himself. Of course, let us not forget that Lear has just chosen to divide his kingdom up between Goneril and Regan, his two daughters, on the condition that they alternately will host him and his train of followers. Note the full speech that Lear utters as he reflects on ingratitude and how Goneril has treated him:
Ingratitude, thou marble-hearted fiend,
Mroe hideous when thou show'st thee in a child
Than the sea monster!
Lear, in this apostrophe, addresses ingratitude, describing it as a "marble-hearted fiend," perhaps referring to the way that its heart is coated in marble and so no one can penetrate it so that it has no conscience or feelings of remorse. Of course, seeing ingratitude in your own child is far more worse than seeing it in a monster that you expect to show ingratitude. Perhaps Lear is here beginning to understand and reflect on his mistake in banishing Cordelia so rapidly.