"effeminate man, homosexual," 1904, from female name Nancy (q.v.), which was in use as an adjective meaning "effeminate" (applied to men) from 1883, a shortening of earlier Miss Nancy.
Nancy, Miss, an opprobrious epithet for an exceedingly effeminate, over-nice young man. The original Miss Nancy, however, was a Mrs. Anna Old field, a celebrated actress, who died in 1730 and was buried in Westminster Abbey. She was extremely vain and nice about her dress, and as she lay in state, attended by two noblemen, she was attired, as she had directed shortly before her death, in "a very fine Brussels lace head-dress, a Holland shift with a tucker and double ruffles of the same lace, a pair of new kid gloves," etc., a circumstance alluded to by Pope .... [William S. Walsh, "Handy-Book of Literary Curiosities," 1892]
Nancy boy "effeminate male homosexual" is attested by 1958.