I appreciate this discussion because I, too, wonder about boredom when it shows up. Just in general, even when I’m not meditating. Wikipedia describes the etymology like this:
The noun “bore” comes from the verb “bore”, which had the meaning “[to] be tiresome or dull” first attested [in] 1768, a vogue word c. 1780–81 according to Grose (1785); possibly a figurative extension of “to move forward slowly and persistently, as a [hole-] boring tool does.”
The Wikipedia entry then links this to the practice of being a bore, i.e., boredom.
Interestingly, in French, the analogous term ennui derives from Latin inodiare (“to make loathsome”) - to Old French anoier or ennuyer (which augments the meaning to include annoyance) - to the Modern French noun ennui. This modern French term also connotes a kind of existential boredom which, in my view, the English term doesn’t convey.
The Pali reads like this, in reference to the third hindrance:
Atthi, bhikkhave, arati tandi vijambhitā bhattasammado cetaso ca līnattaṁ.
Translated by Bhikkhu Bodhi: There are, bhikkhus, discontent, lethargy, lazy stretching, drowsiness after meals, sluggishness of mind…
(Bhikkhu Sujato also translates arati as discontent.)
The Pali term arati is the negation of the noun rati, which means a liking (for something) per the Digital Pali Dictionary.
So if we bring this all the way back to the object of this discussion, I question whether there is a Pali word that is exactly analogous to the English term boredom or the French term ennui. The etymologies above reflect, in my opinion, an averse state of mind or mood (consistent with what @paul1 wrote). I have often wondered whether our current English term boredom is a product of modern culture … if we’re not activated by something in the moment, then we’re averse to that; if we don’t see purpose/meaning aligned to what we’re doing at points in time, then we’re averse to that (more existentially the French meaning). And so on.
Whether nesting boredom under byāpādassa (the literal word in the second hindrance) or under arati tandi vijambhitā bhattasammado cetaso ca līnattaṁ (the third hindrance) may be of less interest to me as the years go on. More recently I tend to observe a bored mind as a restless mind.
Anyway, the etymology research (thanks for the impetus @Soren ) helps me.