brief:  Brief comes via Old French bref from Latin brevis ‘short’, which is probably related to Greek brakhús ‘short’, from which English gets the combining form brachy-, as in brachycephalic. Latin produced the nominal derivative breve ‘letter’, later ‘summary’, which came into English in the 14th century in the sense ‘letter of authority’ (German has brief simply meaning ‘letter’).
The notion of an ‘abbreviation’ or ‘summary’ followed in the next century, and the modern legal sense ‘summary of the facts of a case’ developed in the 17th century. This formed the basis of the verbal sense ‘inform and instruct’, which is 19th-century. Briefs ‘underpants’ are 20th-century. The musical use of the noun breve began in the 15th century when, logically enough, it meant ‘short note’.
Modern usage, in which it denotes the longest note, comes from Italian breve. Other derivatives of brief include brevity , introduced into English via Anglo-Norman brevete; abbreviate , from late Latin abbreviāre (which is also the source, via Old French abregier, of abridge ); and breviary ‘book of church services’ , from Latin breviārium. => abbreviate, abridge, brevity
late 13c., from Latin brevis (adj.) "short, low, little, shallow," from PIE *mregh-wi-, from root *mregh-u- "short" (cognates: Greek brakhys "short," Old Church Slavonic bruzeja "shallow places, shoals," Gothic gamaurgjan "to shorten").
from Latin breve (genitive brevis), noun derivative of adjective brevis (see brief (adj.)) which came to mean "letter, summary," specifically a letter of the pope (less ample and solemn than a bull), and thus came to mean "letter of authority," which yielded the modern, legal sense of "summary of the facts of a case" (1630s).