"Which do you mean?" and turning round, he looked for a moment at Elizabeth, till catching her eye, he withdrew his own and coldly said, "She is tolerable; but not handsome enough to tempt me; and I am in no humour at present to give consequence to young ladies who are slighted by other men. You had better return to your partner and enjoy her smiles, for you are wasting your time with me." (Chapter 3)

Darcy is the only gentleman who slighted Lizzy at the assembly ball—he is acting very ungentlemanly both by not dancing and by making this remark. His conclusion that she is slighted by other men is not true. The real reason Lizzy was sitting down is given a few paragraphs before Darcy’s rude comment:

Elizabeth Bennet had been obliged, by the scarcity of gentlemen, to sit down for two dances; and during part of that time Mr. Darcy had been standing near enough for her to overhear a conversation between him and Mr. Bingley, who came from the dance for a few minutes, to press his friend to join it. (Chapter 3)

Lizzy is not slighted by other men. Darcy insults her appearance and denigrates her consequence because he is irritated with Bingley’s efforts to stop him standing about in a stupid manner. Darcy had no basis for his comment except that Lizzy was sitting down. (;D)

I have linked a post by Julie W on dancing etiquette for gentlemen made earlier in the group read. It shows why Darcy was remiss in not dancing. ;D