Ah yes, prognostication. Such a rare word that it's only uttered multiple times in one of the most popular Bill Murray movies made which is guaranteed to be on TV at least once a year. Seriously, that's not even nearly one of the best words.
The purpose of a dictionary is to aid you in understanding the meaning any English speaker or writer is trying to convey independent of the time period or location, not to dictate valid and invalid words. Some words might be dropped from abridged dictionaries when they fall out of popularity but in 'The English Dictionary' (which doesn't really exist but is approximated by dictionaries like Oxford's) words are added but never removed.
This is different, than say, the mission of L'Académie française which is to act as a language regulator and maintain the official vocabulary and grammar of French.
Sesquiedalian seems to have been popping up since the 1600s.
http://www.oed.com/view/Entry/176752?redirectedFrom=sesquipe...