[H]e asked her if she would do him the honour of introducing him to her friends. This was a stroke of civility for which she was quite unprepared; and she could hardly suppress a smile at his being now seeking the acquaintance of some of those very people against whom his pride had revolted in his offer to herself. "What will be his surprise," thought she, "when he knows who they are? He takes them now for people of fashion." (Ch. 4)

From Johnson's dictionary:
fashion: rank; condition above the vulgar
fashionable: having rank above the vulgar

Elizabeth smiles because she is sure that Darcy believes that the Gardiners are people of rank, not the lower connections that he had despised.

Mrs. Hurst, who had married a man of more fashion than fortune (Ch. 4)