Blaise Pascal, the French mathematician and philosopher, at the end of the sixteenth of his Provincial Letters, dated 4 December 1656: “Je n'ai fait celle-ci plus longue que parce que je n'ai pas eu le loisir de la faire plus courte” — I made this one longer only because I did not have the leisure to make it shorter. The joke has since been credited to Mark Twain, T. S. Eliot and others.
What it means
Being brief takes more work than being long. A first draft runs on; cutting it down to what matters is the slow part, so a long letter can be a sign of haste rather than effort.
Where it comes from
The Provincial Letters were written during the dispute between the Jansenists and the Jesuits in the 1650s. Letter XVI was hurried out, and Pascal apologises for its length. In Thomas M'Crie's English translation: “The present letter is a very long one, simply because I had no leisure to make it shorter.” M'Crie's introduction explains that the sixteenth letter was published in a hurry because the printing office was being searched.
Wikiquote gives the French original and the date, 4 December 1656, and notes that Quote Investigator concludes Pascal's sentence is the likely source of the many later versions credited to Mark Twain, T. S. Eliot and others.