• (RP, GA) IPA: /ˈfiːndɪʃli/

fiendishly

1. In a fiendish manner; evilly, wickedly.
   • 1832, Jedadiah Cleishbotham [pseudonym; Walter Scott], chapter X, in Tales of My Landlord, Fourth and Last Series. [...] In Four Volumes, volume I (Count Robert of Paris), Edinburgh: Printed [by Ballantyne and Company] for Robert Cadell; London: Whittaker and Co., OCLC 81177709 ↗, page 306 ↗:

     At a side door, reclined on a couch, two guards of the haram, with their naked swords grasped in their hands, and features, fiendishly contorted between sleep and dissolution, seemed to menace death to any who should venture to approach.

2. Extremely, very.

• devilishly
• diabolically
• (in a fiendish manner) satanically

• fiend
• fiendish
• fiendishness

• French: diablement
• Italian: diabolicamente