adj. Sinister; evil; conniving; in the manner of a fiend.
the GNU version of the Collaborative International Dictionary of English
adj. Like a fiend; diabolically wicked or cruel; infernal; malignant; devilish; hellish.
The Century Dictionary and Cyclopedia
Having the qualities of a fiend; characteristic of a fiend; demoniacal; extremely wicked, cruel, or malicious; devilish: as, a fiendish persecutor; fiendish laughter.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. extremely evil or cruel; expressive of cruelty or befitting hell
Word Usage
"It was a high-pitched laugh and if a sound ever deserved to be called fiendish that one did."