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2.
Pronounced with the highest voice possible. It is said when the circumstances or actions of a person are extremely ridiculous. The history of this word comes from when Thomas' mum, in response to the question, "Does Thomas have a girlfriend?" answered "No, Thomas thinks girls are silly!" This story has evolved into one where Thomas' mum says the sentence extremely quickly and with a very high voice, hench the pronounciation of silli. Silli can be used in addition to other words that sound like or ryhme with silli eg. 'willy' or 'little'as long as the other words are said in a high and fast tone.
1.
Jim:"Shit, Miss Kent just blew up the chemistry department becuase she mixed potassium phosphate with magnesium nitrate!"

John: "Silli!"

2.
Jim: "Did you hear what James did? He tried to break into Billy's house but he went to a council flat in Balham!"

John: "Thats a little bit silly becuase Billy lives in a pavillion in Picadilly!"
by Phaidan October 22, 2006
 
1.
Chaldean or Arabic word for "i just fucked". Usually followed by the word "bro"
Chaldean 1: Silli Broooo!!!!
Chaldean 2: Me too! Erub cuta!!!
by BLGFTW December 18, 2009