"You canker blossom!" 3 Shakespearean Insults
1953, in hipster slang, from un- (1) "not" + slang sense of cool (adj.).
A black man who emulates or adopts the behavior of the white majority; a servile black man; afro-saxon, oreo
[1922+ Black; fr the title character of Harriet Beecher Stowe's novel Uncle Tom's Cabin, who was stigmatized as a dishonorably submissive black]