A mistress, paramour, or prostitute.
The word’s ultimate origin is uncertain but the Oxford English Dictionary suggests a connection with dock meaning ‘buttocks’ or ‘tail’. As a verb dock originally meant to cut off a tail, a sense that we still preserve when we talk of docking a horse’s or dog’s tail. (The more general idea of cutting or reducing, as when we talk of docking someone’s pay is a later extension of this idea.)
– Wordsworth Dictionary of Obscenity and Taboo by James McDonald.