sis (s?s)n. Informal Sister.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.sis (s?s) ninformal short for sistersis (s?s; si?s) or siesinterjinformal South African an exclamation of disgust[Afrikaans, possibly from Khoi]SIS abbreviation for 1. (Government, Politics & Diplomacy) Also called: MI6 (in Britain) Secret Intelligence Service 2. (Government, Politics & Diplomacy) (in New Zealand) Security Intelligence Service Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014sis (s?s) n. sister. [1825?35, Amer.; shortened form] -sis a suffix appearing in loanwords from Greek, where it was used to form from verbs abstract nouns of action, process, state, condition, etc.: aphesis; thesis. [