Abbr. for 'stupid idiot'
As in Football Factory from 2004:
"Jog on, you si!"
by Nige Menzel May 24, 2007
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
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
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
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
when text messaging, it is used for the idiots who don't understand that you're being sarcastic; means "sarcasm implied," and is often used at the end of a text message.
Janet: "Do you think Miguel was being serious?"

Miguel's text: "yeah i'm really excited to see you too (s.i.)"

Ted: "Of course not, he even put 'sarcasm implied' in the text! You must be really dumb"
by upsidedownchoking December 03, 2009
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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