From "The Concise SGML Companion Glossary (Neil Bradley), http://www.bradley.co.uk/Pages/Topic_SGMLCompanion.htm :
From "A Gentle Introduction to SGML," edited by C. M. Sperberg-McQueen and Lou Burnard (http://etext.virginia.edu/bin/tei-tocs?div=DIV1&id=SG):
Might also stand for "Sounds Good, Maybe Later", a joke referring to SGML's intimidating complexity.
Created originally in 1969. See The SGML History Niche for details of SGML's early development. http://www.sgmlsource.com/history/
Try the ExtensibleMarkupLanguage (XML), which is less complicated. XML was originally intended to be a 'simplification' of SGML (80% of the power with 20% of the complexity).