Green’s Dictionary of Slang

an effeminate youth, a homosexual.

1889–90		Barrère & Leland Dict. of Sl., Jargon and Cant II 81/1: Nancy Dawson (popular), a name for a molly, an effeminate youth, apathetic, &c. A recent sketch of the characteristics of the mashers of the present day [...] represents two of the fraternity, who are very intimate, always calling one another by girls’ names. [...] The original Nancy Dawson was a noted prostitute, on whom there is still a song current among sailors.
a.1889		‘Nancy Dawson’ [ballad] They call him Nancy Dawson, / And isn’t he a caution! / Oh, Mr Nancy Dawson, what a tricky man you are! / Oh, Nancy Dawson, can’t you do the lar-di-dar?