An unbelievably rich cunt (richer than Bill Gates) who wears a monacle and probably has other characteristics but none you really care about.

Also likes boys and sniffing the sweatline under fat guys flabbing man breasts.
Brian has loads of money for beer tonight. He could do a Si and buy the pub free drininks all night.
by Bobby Charlton March 18, 2004
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Due to the vast majority of lies which happens every time a person types 'LOL' in texting or the internet, SIS (Smile in Silence) is a much more proper use in times when one does not actually laugh when typing 'LOL'. Though, this may suggest that the person is not as fun as he did not actually laugh.
A: Yo, B, I just tried walking thru at a drive thru.
B: LOL, what happened?
A: B, stop lying bro, don't tell me you actually laughed at that message. SIS
B: Jeez, such a stickler...So what happened?
A: They told me that they only viewed car owners as acceptable customers.
B: SIS
by Shinigami Prophet July 11, 2014
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sexual intercourse sex hanky panky hot hottie awesome cool sexy 1. a word and hand-sign created to refer to sexual intercourse in a way unknown to rejects, spazes, and especially older, uncool adults, so your conversations remain esoteric.

2. this term can take the place of words such as hot, hottie, sexy lady, ect.

3. SI can also replace cool, awesome, ect.
ex1: Things are really going well with my gf. I think SI is an inevitability.

ex2: Whoa! Check her out! SI at 10 o'clock.

ex3: "Hey, there having free food at the party, and there's no addmission price this time!"

"Really? Aw, SI man!"
by Oddjob 007 September 26, 2006
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Socially Inept.

Seen in episode 13 of "Rugrats: All Grown Up"
"Lil, your brother is what he called SI, you know, socially INEPT"
by Johnny Skeets May 30, 2005
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derogatory term for man who thinks hes a fox.
si, you isnt a fox dude!
by kezza January 31, 2005
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Also Spelt Sharn.
si-an is a slang MSN term
for big chin
it also consists of a bob blonde boy cut
and 'thinking your indie'
Taking wrongly Shaped bebo photos covering your massive chin.
-SIIIII-AAAAAAAAAAN-
-whos? got a si-an?-
-looks at bebo picture-
-massive chin-
by Dropdeadchloe January 14, 2008
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