The latest uncommon word I have encountered is nudiustertian. No, it's not an Austrian word to describe a nude citizen. It's the proper term to describe the time frame pertaining to the day before yesterday.
I think it's an important word because it's the pivotal point in history where memory starts to go fuzzy. Remembering what happened yesterday is fine but the day before... may as well be the year before yesterday. Alternatively if you drink a lot it's where your last memory starts since you woke up.
I couldn't find a picture I liked depicting the day before yesterday, so here's one of some nude Austrians.