-sis a suffix appearing in loanwords from Greek, where it was used to form from verbs abstract nouns of action, process, state, condition, etc.: thesis; aphesis.
Origin: < Gk
sis Informal
The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2009 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Abbreviations & Acronyms
SIS
The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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[sis] 
