Green’s Dictionary of Slang

fair roebuck n.

[SE fair roebuck, a roebuck in its fifth year]

a woman at the peak of her beauty.

[UK]New Canting Dict. n.p.: fair-Roe-Buck [...] in a Canting Sense, a Woman in the Bloom of her Beauty.
[UK]Bailey Universal Etym. Eng. Dict. [as cit. 1725].