It's just the adverbial form of sneak or sneaky. It's not a real word and it damned well should be!
I went into Dr. Aweful's mountain lab sneakily, and stole his blueprints for a flying monkey machine!
by Felix January 09, 2005
I followed bob sneakily around the building and scaried him.
by Blackmoondust November 13, 2009
The act of being sneaky. Performing actions in a sneaky way.
He was sneakily sending text messages to his mistress as his wife watched the kids.
by PappaZappa April 03, 2009