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.