I can't fathom this is the oldest, but you might be interested in the evolution of acedia (Greek ἀκηδία). The Greek meaning is originally "negligence," but late antique monastic writers turn it into the genesis of neglecting monastic duties: "spiritual listlessness."

Through Cassian and then Latin medieval writers, it becomes one of the seven deadly sins. You know it today as "sloth" (it acquires the modern interpretation of laziness especially through sermons directed at urban classes during the late Middle Ages and early modern era, with the ramping up of artisan labor/proto-industrialization).

Acedia itself is more spiritualized or existential than modern "boredom." Elsewhere on AskHistorians I've suggested that it overlaps (but does not mirror precisely) the modern idea of depression. Writes monk John Cassian in the early 5th century:

He looks about anxiously this way and that, and sighs that none of the brothers come to see him, and often goes in and out of his cell, and frequently gazes up at the sun, as if it was too slow in setting, and so a kind of unreasonable confusion of mind takes possession of him like some foul darkness.

My very favorite example of historical existential angst is Chaucer's poems where he has his poet-narrator (autobiographical? it's deliberately ambiguous) angst about how he's supposed to be this great poet of romance revealing the deepest truths about love and the heart of the human condition...BUT HE'S NEVER BEEN IN LOVE.