Английский Этимологический словарь - brief (adj.)

- 13c., from L. brevis (adj.) "short." Noun derivative breve (gen. brevis) meant "letter, summary" (specifically a letter of the pope, less ample and solemn than a bull), and came to mean "letter of authority," which yielded the modern, legal sense of "summary of the facts of a case" is 1631.