batten [ˈbæt(ə)n] v. i.
1.) To grow better or improve in condition; especially (of animals) to improve in bodily condition by feeding, to feed to advantage, thrive, grow fat.
2.) To feed gluttonously on, glut oneself; to gloat or revel in. (With indirect passive, to be battened on, in modern writers.)
3.) fig. To thrive, grow fat, prosper (especially in a bad sense, at the expense or to the detriment of another); to gratify a morbid mental craving.
4.) To grow fertile (as soil); to grow rank (as a plant) (Oxford English Dictionary 2nd Edition).
Etymology: First found in end of 16th century, but may have been in dialectal use before; apparently adopted from Old Norse batna to improve, get better, recover, from bati, advantage, improvement, amelioration; cognate with Gothic gabatnan, to be advantaged, to be bettered, to profit, a neuter-passive form derived from batan, bôt, batans, to be useful, to profit, to boot. Cf. also Dutch baten, to avail, yield profit; baat, profit, gain, advantage, benefit; and see Grimm s.v. batten. A cognate bat in sense of 'profit, advantage, improvement,' although not known as a separate word in English, is implied in the derivatives batt-able, bat-ful, batt-le (a.).
"The Medical College piles up in its museum its grim monsters of morbid anatomy, and there are melancholy skeptics with a taste for carrion who batten on the hideous facts in historypersecutions, inquisitions, St. Bartholomew massacres, devilish lives, Nero, Caesar Borgia, Marat, Lopez; men in whom every ray of humanity was extinguished, parricides, matricides and whatever moral monsters. These are not cheerful facts, but they do not disturb a healthy mind; they require of us a patience as robust as the energy that attacks us, and an unresting exploration of final causes" (Society and Solitude, Ralph Waldo Emerson, 1870).

(Pochodnie Nerona, Henryk Siemiradzki, 1876)
dernier [ˈdɜːnɪə(r)] a.
1.) Last; ultimate, final.
2.) dernier ressort: last resort; originally (in reference to legal jurisdiction) the last tribunal or court to which appeal can be made, that which has the power of final decision; hence, a last or final resource or refuge.
3.) dernier cri [French, literally 'the last cry']: the very latest fashion. Also in predicative use and (without article) attributively.
4.) dernier mot: the last word (Oxford English Dictionary 2nd Edition).
Etymology: adoption of French dernier, last, latest.
"The Right Honourable William Humble, earl of Dudley, G. C. V. O., passed Micky Anderson's alltimesticking watches and Henry and James's wax smartsuited freshcheeked models, the gentleman Henry, dernier cri James" (Ulysses, James Joyce, 1922).

(The Fighting Temeraire tugged to her last berth to be broken up, Joseph Mallord William Turner, 1839)_____________________________________________________
I'm pleased to report that this word finally allowed me to find a mistake by my hero David Foster Wallace. See, I originally discovered this word when I was rereading his essay "Up, Simba" the other month (and wrote it down in my trusty moleskin to save for later, as is my wont). He writes:"One of the scrum's oldest and most elite 12M calls out one last time that surely after all there aren't any guns to the candidates' heads in this race, that surely Mike (the Monkeys call him Mike) would have to admit that simply refusing to 'quote, "respond"' to Bush and thereby 'staying on the high road' was something McCain could have done; and Murphy's dernier cri, over his shoulder, is 'You guys want a pacifist, go support Bradley.'"
But he should have written dernier mot, as anyone can see. Probably he was relying on his knowledge of French. But dernier cri doesn't mean last cry in English, it means latest fashion. So, once again, not knowing French turns out to be a better strategy in life.
ad kalendas graecas [ɑd kɑˈlɛndɑs ˈgraɪkɑs] adv.
1.) Never (Dictionary of Foreign Words, Adrian Room, 2000).
Etymology: Latin, literally on the Greek calends, from ad + kalendas graecas, accusative of kalendae graecae, Greek calends. The Greeks had no calends in their calendar, so this refers to a nonexistent time. (In the Roman calendar the calends were on the first day of any month.) Suetonius reports in his Life of Augustus that the emperor coined the phrase with reference to people who never planned to pay their debts: he would say "ad Kalendas Graecas soluturos" ("they will pay on the Greek Kalends").
"The dominion of Christ does not appearin complete contrast to the Roman Empireas an obvious, earthly and present political power. Thus the earthly power, ruling here and now, has no needin its delusionto feel concerned; the 'end of the age' and the coming of Christ for judgement can be deferred, as it were, ad kalendas Graecas" (Studies in Early Christology, Martin Hengel, 2004).

(Η Πέμπτη Σφραγίδα της Αποκαλύψεως, Δομήνικος Θεοτοκόπουλος, ~1611)________________________________________________________
Apparently there's the equivalent English phrase, "on the Greek calends," as well. That phrase is in the OED, while the Latin phrase isn't. But if you're going to make a Roman joke, I figure you might as well do it in Latin.