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1.
One of the best chapters in the order of the Aleph Zadik Aleph, SiWi is short for Simon Wiesenthal AZA. Hailing from the cities of Palo Alto, Los Altos, and Mountain view, SiWi has often been reffered to as one of the powerhouses of the California Bay Area.

Originally founded in November of 2005, to this day Simon Wiesenthal AZA remains an amazing chapter.
Dude, SiWi is by far the best chapter!
by GuyAZA March 15, 2011
 
2.
SIWI is an abbreviation, that stands for the phrase 'so I won it'.
While you are playing with your friend rock-stone-scissors, after score you may shout 'SIWI'
by PinkSourcy September 27, 2013