Our little girl Susan is a most admirable slut, and pleases us mightily. [Pepys, diary, Feb. 21, 1664]
Sometimes used 19c. as a euphemism for bitch to describe a female dog. There is a group of North Sea Germanic words in sl- that mean "sloppy," and also "slovenly woman," and that tend to evolve toward "woman of loose morals" (cf. slattern, also English dialectal slummock "a dirty, untidy, or slovenly person," 1861; Middle Dutch slore "a sluttish woman").