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1.
Silently Smiling To Myself. A more subtle form of lol, as opposed to laughing out loud, one simply enjoys a quiet grin. The acronym can stand for "Smiling Silently..." as well.
AveragePersonA: Check out this funny video, lol!
AveragePersonB: Yeah... SSTM.
by ubers July 04, 2010
 
2.
Smuggly Smiling To Myself
Person 1: Camden Market is burning!
Person 2: SSTM! :P
by STEVENAGE ROGER February 09, 2008