Green’s Dictionary of Slang

a cheap whore, likely to be infested with crab-lice.

1674		Rochester ‘Tunbridge Wells’ in Works (1999) 49: To purchase some inspiring pretty Toy / That may his want of sence, and witt supply / As Buxom Crabfish do’s his Lechery.
1992		Randolph & Legman Ozark Folksongs and Folklore I 4: The fine text of ‘The Crab-Fish’ ballad [...] collected by Sharp from a woman singer, almost at the end of his career in 1921; first recorded in England as ‘The Sea Crab’.