This train of prefixes surely needs uncoupling. Something that is ultimate is the last in a series (from Latin ultimare, come to an end); the penultimate is next to last (pen-, a prefix from Latin paene, almost); the antepenultimate is the one before that (ante-, previous, from Latin “ante”). Preantepenultimate (Latin prae-, before) is one step further back still, making it the fourth from the end of the series, the last but three.
It was invented in all seriousness by a famous lexicographer, John Walker, in his Critical Pronouncing Dictionary and Expositor of the English Language, dated 1791 (“These words have the antepenultimate and preantepenultimate accent, which has generally a shortening power, as in privilege, primitive, prevalency, &c.”) Linguists have continued to be almost its sole users (though not often) but other specialists, for some reason mainly zoologists, have borrowed it from time to time. Outside these areas, it is almost invisible, but not quite:
While it was gratifying to see the return of the John H. Rice column to the pages of the Eagle, it was disappointing to have a typographical error mangle the sense of his pre-antepenultimate paragraph.
Berkshire Eagle (Pittsfield, Massachusetts), 24 Feb. 1977.
Does the sequence stop with preantepenultimate? There must be so little need for a word meaning fifth from last that we can hardly imagine anybody has taken the trouble to invent it. So it’s surprising to learn that at least two people have had a go. A humorous column in The Beechwood Reporter of Chicago, dated January 2013, mentioned suprapreantepenultimate (Latin supra, above, beyond). A much earlier writer invented another version:
In its minute divisions, accent is ultimate, penultimate, antepenultimate, preantepenultimate, propreantepenultimate.
A General Critical Grammar of the Inglish Language on a System Novel and Extensive, by Samuel Oliver, 1825. His extra prefix is from Greek pro, before; Inglish was his personal way to spell English.
Both of these are unwieldy, even monstrous — it’s far easier just to say “fifth from last”.