Green’s Dictionary of Slang

a prostitute.

c.1699		N. Ward ‘A Trip to New England’ in Writings (1704) 180: I am sure a Covent-Garden Beau, or Bel-fa would appear to them much stranger Monsters, than ever yet were seen in America.
1699		N. Ward Walk to Islington 64: Beaus with their Belfa’s were Cringing and Talking.
1700		N. Ward London Spy XVI 392: The Beaus began to Rise and come forth [...] in order to attract the Eyes of some Mercenary Bellfa’s. [Ibid.] 395: The Bellfa’s, in their Morning Gowns, and Wadded Waste-Coats, without Stays, began to flow as fast into the Walks, as Whores into the Eighteen-Penny Gallery, at the third Act.
1708		Almanack-Husband 6: [He] espies a flaming Bilfa [...] and was [...] smitten with her killing Ogles.