Pride & Prejudice, Chapter X of Volume I (Chap. 10)
"It is a rule with me, that a person who can write a long letter, with ease, cannot write ill."
"That will not do for a compliment to Darcy, Caroline," cried her brother -- "because he does not write with ease. He studies too much for words of four syllables. -- Do not you, Darcy?"
"My stile of writing is very different from yours."
"Charles writes in the most careless way imaginable. He leaves out half his words, and blots the rest."
"My ideas flow so rapidly that I have not time to express them -- by which means my letters sometimes convey no ideas at all to my correspondents."
"Your humility, Mr. Bingley," said Elizabeth "must disarm reproof."
The lines quoted are taken from a conversation between Caroline Bingley, Mr. Darcy and Mr. Bingley and, later, Elizabeth. At first, Caroline is flattering Darcy in order to gain his attention and affection by manufacturing artificial compliments to his way of writing a letter since he is occupied at the moment in writing a letter to his sister.
Caroline says that anyone who writes a long letter must also be a talented letter writer. Bingley dissents and says that can't apply to Darcy because Darcy only takes a long time writing because he is meticulous and tries to find long four syllable words. This is a good matured dig at Darcy's habitual formality.
Caroline then says that Mr. Bingley writes careless letters with words omitted and ink blotches covering other words. Bingley then utters the quoted line. What he means to say is that he thinks so quickly that his hand can't keep up with his mind. Austen introduced this brilliantly witty conversation, which continues to "Sunday night..." to reveal the characters' psychological makeup and the dynamics between relationships, also to encourage the antagonism between Elizabeth and Darcy and Elizabeth and Caroline Bingley.
The remark about "Sunday evening" with "nothing to do" is a playful jab at Darcy's temperament and an effort to divert the conversation away form an arguing debate about whether friends ought to be easily persuaded by each other: "I see your design, Bingley,'' said [Darcy]. -- "You dislike an argument, and want to silence this."