|Noah Webster's Dictionary|
(adv.) Secretly; in private; insidiously.
Covertly (2 Occurrences)
Genesis 31:27 Why didst thou flee away covertly, and steal away from me, and didst not tell me, that I might have conducted thee with mirth and with songs, with tambour and with harp; (DBY)
2 Kings 17:9 and the sons of Israel do covertly things that 'are' not right against Jehovah their God, and build for them high places in all their cities, from a tower of the watchers unto the fenced city, (YLT)