Pronounced zih-zih-vah, this word is accredited/associated with 4 things (2 of them widely accepted/known, 1 not so widely known, and 1 personal opinion, you try and pick which one):

1. Being the last word in the English Dictionary (aside from street names and zzz, the term used by many people to described a drawn character who is sleeping).

2. One of the weirdest spelt words known to man.

3. So says the American Heritage Dictionary:

NOUN: Any of various tropical American weevils of the genus Zyzzyva, often destructive to plants.

ETYMOLOGY: New Latin Zyzzyva, genus name, probably from Zyzza, former genus of leafhoppers.

4. The name of a literary magazine, based in San Francisco, that publishes writing and art by west coast writers and artists
Sources:
The Last Word (http://www.zolid.com/lastword/)
The American Heritage Dictionary 4th Edition Definition (http://www.bartleby.com/61/30/Z0033000.html)
Zyzzyva.org