Green’s Dictionary of Slang

(US black) excellent, wonderful, admirable.

1905		Wash. Post 15 Jan. 4/3: One bright, beautiful girl [...] uses the word ‘fiendish’ to express the superlative degree of everything.
1952		H.H. Griffin the Devil rides outside 379: ‘Wish I had a bellyful of that one myself. Was it good?’ ‘She’s positively fiendish, Salesky’.
1968		‘Sl. of Watts’ in Current Sl. III:2 23: Fiendish, adj. Attractive; good; more than words can explain.
1980		E. Folb Runnin’ Down Some Lines 120: Like many other synonyms for fine and exceptional in black vernacular (fiendish, terrible, bad, tough, mean, wicked), [it] inverts standard white usage for the words. Bad becomes good.