Elizabeth had never been more at a loss to make her feelings appear what they were not. It was necessary to laugh, when she would rather have cried. Her father had most cruelly mortified her, by what he said of Mr. Darcy’s indifference.

– Jane Austen

Pride and Prejudice, Chapter 57. Mr. Bennet doesn’t take seriously Mr. Collins’s letter suggesting a marriage between Elizabeth and Mr. Darcy. He speaks insensitively about Darcy’s indifference to his daughter and Elizabeth’s dislike of Darcy. He jokes whether Lady Catherine has called to refuse her consent. Elizabeth goes along with the joke and replies with a laugh, but she feels like crying. The irony here is that Mr. Bennet is completely unaware of his daughter’s true feelings for Darcy.