Short abbreviation for "Stealth Assassin". Mainly used when describing a situation that includes a person or object sliding past someone's point of view quickly and almost unnoticed. Or, a person who mainly uses moves or spells that make them stealth, such as a Rogue. Another way to use "Stassin" is when you are standing in the AH (World of Warcraft) and another person quickly runs by your screen.
"Dude, I almost got ganked by a stassin"
"Did you see that stassin?"