Here Mr. Darcy is alluding to William Shakespeare's Twelfth Night. This comedy tells the story Viola, a women who must dress like a man
Jane Austen makes a sly reference to false first impressions. In Twelfth Night, the female protagonist, Viola, must dress as a man while serving her master Duke Orsino. She falls inlove with the Duke, but the feeling is not reciprcated due to her manly disguise. At first Jane accuses Mr. Darcy's of having a cold countenance and proud nature, when in reality he is kind and modest. Duke Orsino's quick judgement is parallel to Jane's.
~"Twelfth Night Plot Synopsis." Shakespeare Online. 02 Apr. 2009