expression of boredom, by 1906. As an adjective, by 1956.
"be tiresome or dull," 1768, a vogue word c. 1780-81 according to Grose (1785); see bore (n.2). As "cause boredom to," by 1840.
by 1884, from French mal du sìecle, "world-weariness, atrophy of the spirit, aristocratic boredom, deep melancholy over the condition of the world," supposedly a characteristic condition of young romantics in Europe in the early 19c. It answers to German Weltschmerz.
1778, "thing which causes ennui or annoyance by dullness;" earlier "state of boredom, fit of listless disgust" (1766); of persons who cause boredom by 1812; usually said to be a figurative extension of bore (v.1) on the notion of "move forward slowly and persistently," as a boring tool does, but OED has doubts and early evidence suggests a French connection.
Le secret d'ennuyer est celui de tout dire (The secret of being a bore is to tell everything) [Voltaire, "Sept Discours en Vers sur l'Homme," 1738]