Green’s Dictionary of Slang

jenny darby n.

a policeman.

1830		Morn. Post (London) 12 Aug. 4/3: The officer instantly seized the war-like watchman [who] called aloud to the crowd, ‘Here’s the b— Jenny Darbys; we shall all be pulled and destroyed by the Jenny Darbys’ — (Laughter) [...] and although he called them by their French name, the Jenny Darbys, it was wonderful how the common people, who knowed little or no French, understood his meaning.
1860		C. Martell Detectives’ Note Book n.p.: Well, I joined the [police] force. [...] There was a good deal of animosity against us for a long while and all sorts of opprobrious epithets were bestowed upon us. We were ‘Bobbies’, ‘Bluebottles’, ‘Peelers’, and ‘Jenny Darbies’ (gens d’armes).