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1.
sis
1) Short for sister
2) In the song Fuck the Pain Away by Peaches, SIS seems to stand for Stay In School...
My sis went to the movies with a scumball.

"Stay in school... Cause it's the best..."
by wxynot July 12, 2005
 
2.
sis
Slang for sister
Me and my sis went to the movies.
by Thunderchild March 25, 2003
 
3.
sis
1.) A South African English expression roughly equivalent to the American "yech" and the Afrikaans expression "sies".

2.) An American term of endearment for a sister, now also used in other countries.
1.) Sis, man, you stink!

2.) Hey, sis, hey bro, how's Ma doin'?
by Xaviana December 01, 2003
 
4.
SIS
Smirk in Silence:
The act of putting on a straight face after putting someone in a spot.
Pulling someone's leg as a group where everyone acts normal in a scenario that is funny.
Used on social networks, chat rooms and forums..at least could be!
Totally loved getting on Spook's nerves acting as if I don't know him at the bar last night. Pulled a SIS.
by far_c November 02, 2009
 
5.
SiS
SiS as in Shit Its Sunday Commonly used on sites like Facebook and Myspace
John: Aw man SiS
Kevin: Dude I know
by Kife1100 April 05, 2009
 
6.
SIS
Snickering in Silence
new for LOL because it applies much more than LOL when texting/typing and trying to be quite
While texting in class i received a message with a prime time to say "thats what she said" and/or "reminds me of my saturday night" or even "I get that all the time" so I sis'd vs lol'ing

"idk what happened it just went everywhere after i touched it"
sis
"reminds me of my saturday night"
by kevin "k-bone" December 25, 2009
 
7.
An acronym for Sudden Insanity Syndrome
Joan Crawford had a lot of experience with S.I.S. moments.
by Candide March 06, 2008