Source : Webster's Revised Unabridged Dictionary (1913)

Slyness \Sly"ness\, n.
   The quality or state of being sly.

Source : WordNet®

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