Accidie is often identified as or equated with Sloth (one of the Seven Deadly Sins). Here’s a little taste of Chaucer’s use of the word in The Parson’s Tale:
After the synne of envye and ire, now wol I speken of the synne of accidie; for envye blyndeth the herte of man, and ire troubleth a man, and accidie maketh him hevy, thoghtful and wrawful. […] Thanne is accidie the angwissh of troubled herte, and Saint Augustyn seith, it is anoy of goodnesse and joye of harm. [And the Parson goes on in much the same prolix manner for quite some while, not unlike myself, I suppose. :)]
In the century before Chaucer, Thomas Aquinas described the sin in his Summa Theologica, equating accidia (the Latin form; also, acedia) with torpor mentis (“a torpor of the mind”), which I think captures the sense of accidie much better than mere sloth. The modern sense of “sloth” (lower-case), implying laziness, seems too dismissive, and too insensitive. (And speaking of sloth, if you had only two toes, you probably wouldn’t get much done either! ;) Torpor, ennui, listlessness, and apathy are all closer to the intended sense than sloth.
Turning to the authority the Parson himself invoked, Augustine of Hippo dissertates on this and the other Deadly Sins in the City of God, but perhaps the best, most thorough description of accidie comes in Book X of John Cassian’s De institutis coenobiorum, written in the early 5th century (contemporary with Augustine).
The Latin accidia is not related to acidus “sour, tart”, nor to accidentia “an accident” (something that befalls one) — these explanations, common enough, are mere folk-etymology. It ultimately springs from the ancient Greek ακήδεια “indifference, torpor”, from α– “not” + κήδος “care”. Interestingly, κήδος eventually became the modern English “hate”. Accidie (the word, yes, but probably the need for the word too, hahae) came to Middle English with the Norman invaders, in the Old Norman French form accidie, acidie (from Old French accide, acide). The word was popular throughout the Middle Ages, particularly in Ecclesiastical use, but all but died out after that.
I say all but died out, because the Inklings and their circle clearly knew the word. John Wain, reviewing C.S. Lewis’s An Experiment in Criticism, referred to “the cultured accidie in Oxford” . Another Inkling, Charles Williams, used the word in more than one of his books (e.g., in The Figure of Beatrice, his study of Dante). W.H. Auden, a student and later friend to Tolkien, wrote a commentary on accidie. Though Lewis does not use the word in it, the Narnia novel, The Silver Chair, deals with the sin of “spiritual sloth” (accidie), and we may be absolutely certain Lewis knew the word. (I believe he may have used the word in his letters, but I will have to check on that; they’re not very, er, portable. :) I’m likewise sure that Tolkien knew the word. It is not in his Middle English Vocabulary, but the man knew his Chaucer, not to mention Aquinas and Augustine.
So. Accidie, then. Less critical than sloth, not so banal as ennui, and well suited to pointed or metaphorical use. Well suited, in short, to be brought back into “the parlance of our times”. Shall we bring it back? Or are we too lazy? ;)
 Qtd in Hooper, Walter. C.S. Lewis: A Complete Guide to His Life and Works. London: HarperCollins, 1996, p. 74. [Published previously as the C.S. Lewis Companion and Guide.]
— Hat tip to Gary for the indirect suggestion of this WOTD. :)