Source : Webster's Revised Unabridged Dictionary (1913)

Sneaking \Sneak"ing\, a.
   Marked by cowardly concealment; deficient in openness and
   courage; underhand; mean; crouching. -- {Sneak"ing*ly}, adv.
   -- {Sneak"ing*ness}, n.

Source : WordNet®

sneakingly
     adv : in a sneaky manner; "I always felt sneakingly that I wanted
           to be a concert pianist"