theftfully--prophetes.ai

† ˈtheftfully, adv. Obs. rare.
  In 5 thift-.
  [f. theft + -ful + -ly².]
  By stealth: = theftly.

c 1400 Sc. Trojan War ii. 1391 Vlixes..frome Troy is passit thiftfully With all þaim of his company.

Oxford English Dictionary