I once forgot the term for a "word that appears only once in a document or official paper". But I remembered "honorificabilitudinitatibus" was one, so I Googled that word and quickly found "hapax legomenon". Yep, probably the most well-known hapax legomenon there is.

Two of my babylon dictionaries define it as follows:

(from yasin's unique words)
honorificabilitudinitatibus - With honour. We are in the arena of sesquipedalian words - those a foot and a half long, whose prime characteristic is their length rather than their sense or usefulness. Any word used both by James Joyce (in "Ulysses") and by William Shakespeare (in "Love's Labour Lost") can't be entirely dismissed from the canon of English, even though the former borrowed it from the latter, who in turn borrowed it from Latin, and it doesn't seem to have been used by anybody else, ever.
Shakespeare's wondrous creation appears in Act 5, Scene 1: I marvel thy master hath not eaten thee for a word;
for thou art not so long by the head as
honorificabilitudinitatibus: thou art easier
swallowed than a flap-dragon.
(Somebody's now sure to ask me about "flap-dragon". It was the name given to a game in which the players snatched raisins out of a dish of burning brandy and extinguished them in their mouths before eating them. By extension, it was the burning raisins used in the game.)"Honorificabilitudinitatibus" is the ablative plural of the Latin honorificabilitudo", honourableness, a long-enough tongue-twister of a word to satisfy most palates by itself. If you're a glutton for punishment, you could also try "honorificabilitudinity", which also means "honourableness".

(from Grandiloquent dictionary)

honorificabilitudinitatibus - With honorableness (a nonsense word from medieval literature)