bulletinyoudaoicibaDictYouDict[bulletin 词源字典]
bulletin: [17] If a bullet is etymologically a ‘little ball’, a bulletin is a ‘little little edict’. It comes via French bulletin from Italian bulletino, which was a diminutive form of bulletta ‘document, voting slip’ (briefly introduced into English in the 17th century as bullet: ‘Elected by the Great Master and his Knights, who give their voices by bullets’, George Sandys, Travels 1615); French billet ‘letter’, and indeed English billet, as in ‘billeting’ soldiers on a house, are parallel formations on a variant of the root of bulletta.

And to return to bulletta, this was itself a diminutive form of bulla, from medieval Latin bulla ‘sealed document’, which is the source of English bull, as in ‘papal bull’.
=> billet, bull[bulletin etymology, bulletin origin, 英语词源]
bulletin (n.)youdaoicibaDictYouDict
1765, from French bulletin (16c.), modeled on Italian bulletino, diminutive of bulletta "document, voting slip," itself a diminutive of Latin bulla (see bull (n.2)) with equivalent of Old French -elet (see -let). The word was used earlier in English in the Italian form (mid-17c.). Popularized by their use in the Napoleonic Wars as the name for dispatches sent from the front and meant for the home public (which led to the proverbial expression as false as a bulletin). Bulletin board is from 1831.