The Collaborative International Dictionary of English v.0.48:
Sneaking \Sneak"ing\, a. Marked by cowardly concealment; deficient in openness and courage; underhand; mean; crouching. -- Sneak"ing*ly, adv. -- Sneak"ing*ness, n. [1913 Webster]WordNet (r) 3.0 (2006):
sneakingly adv 1: in a sneaky manner; "I always felt sneakingly that I wanted to be a concert pianist"