The Collaborative International Dictionary of English v.0.48:

Slyness \Sly"ness\, n.
   The quality or state of being sly.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slyness
    n 1: shrewdness as demonstrated by being skilled in deception
         [syn: craft, craftiness, cunning, foxiness,
         guile, slyness, wiliness]