Green’s Dictionary of Slang

a courtesan, a high-class prostitute.

1750		Love at First Sight 158: [A] certain Gentleman of Fashion and Fortune, lost his Heart [...] and had Intelligence that it was pick’d up by a fair Incognita .
1790		M.P. Andrews Better Late than Never 29: You are going to sigh away the evening with some fair incognita.
c.1850		Peeping Tom (London) 1 2/3: The terms proposed by his incognita were these [etc].
1864, 1867, 1870		Hotten Sl. Dict.