suprisingly improved sequel
a movie is an s.i.s if

a) the original was brilliant,yet the sequel was better
b) the original was average and no one expected an improvment
c) the original was pure garbage and no one wanted a sequel. a monkey could make a better sequel but this sequel was actually really good
aliens and terminator 2 are s.i.s type a imo

mad max 2 is awesome, it is an s.i.s type b because the first one was quite boring

mate, the star wars prequels were such utter shyte, im glad revenge of the sith was better, it was a type c s.i.s
by evan4401 October 02, 2010

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Top Definition
Si
Having a sign language rap battle.
Tommy couldn't hear the shit that Warren was signing about him.
by will bitten December 21, 2016

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SI
Abbr. for 'stupid idiot'
As in Football Factory from 2004:
"Jog on, you si!"
by Nige Menzel May 24, 2007

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SI
Abbr. for "self-injury," also known as self-mutilation. Most common form is cutting, but may also include burning, bruising, skin-picking (excoriation), and other behaviors. There is no single cause for SI. SI is most common in women and tends to persist for years.
As a result of overwhelming anxiety, she turned to SI as a coping mechanism.
by rieux November 27, 2003

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S.I.S is an acronym for Special Investigation Section.

It is an Los Angeles Police Department's elite tactical detective squad with a straightforward mandate: Track down the City’s most dangerous offenders and take them off the street. S.I.S. are possibly the hardest-working, most dedicated men and women Detectives to wear a badge.
Alonzo: Hey, Mark, where are you transferring to?
Mark: S.I.S.
Alonzo: Yeah, S.I.S. Detective.
(from 'Training Day')
by KainJynez June 22, 2009

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SI
stands for self-injury. Aka cutting, burning, etc. for emotional distress reasons
Person 1- Did you SI today?

Person 2- I haven't hurt myself on purpose in like a week.
by Injuried November 07, 2010

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Means to "sit in silence" insetad of lol. Usually used if somebody sais something retarded online, its a good way to be honest. Lets face it very few actually laugh at stuff online.
xXihateyouxX: i was like "duhhhhhhh" haha
ay u guys 2003: s.i.s
by mat a April 24, 2005

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Si
An awesome boy that is kind, nice, derpy, hilarious and clumsy and is a wonderful bestfriend.
Is that Si?!
by kjj-123 December 30, 2012

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