I noticed the use of monstrous pretty several times from sir John and it seemed especially illbred when the Palmers first visit the cottage and Marianne joins them a little later. He cries to his brother in law: "Here comes Marianne! Now Palmer you shall see a monstrous pretty girl."

This is such a lively scene BTW where Sir John comes through the turf to make Elinor open the window and tell him what she thinks of his new guests. She tries to hush him but he declares it is only the Palmers! And he asks if Marianne has run off upon seeing them arrive! Then Mrs Jennings comes hallooing! And goes off to speak all of her mind.
All this by the open window while the unknown guests are waiting by the door. Monstrous entertaining.