From Middle English gilefuli, gylfully, gilfulliche, gylefulliche, equivalent to guileful + -ly.
guilefully (comparative more guilefully, superlative most guilefully)
- In a guileful manner.
- 1596, Edmund Spenser, The Faerie Queene, V.11:
- Sith which she hath me ever since abhord, / And to my foe hath guilefully consented: / Ay me, that ever guyle in wemen was invented!