The Collaborative International Dictionary of English v.0.48:
Furtively \Fur"tive*ly\, adv. Stealthily by theft. --Lover. [1913 Webster]WordNet (r) 3.0 (2006):
furtively adv 1: in a furtive manner; "the soldiers were furtively crawling through the night" [syn: furtively, on the sly]