adverb. as a devil; in an evil manner.
1. Adjective The game takes the graphics and gameplay of that classic title and reworks them into a series of fiendishly difficult levels for advanced players. 2. Adverb It's a lamentable fact that the freshest, best-tasting eggs are fiendishly difficult to peel when they're boiled, and that eggs a week or two older are much easier to work with.