Green’s Dictionary of Slang

a coat with the pockets on the inside, making it harder to pick.

1839		H. Brandon Dict. of the Flash or Cant Lang. 164/2: Patent coat – inside skirt coat pocket.
1857		J. Archbold Magistrate’s Assistant 446: Inside skirt coat pocket – Patent Coat.
1859		Hotten Dict. of Modern Sl. etc. 72: patent coat a coat with the pockets inside the skirt ― termed patent from the difficulty of picking it.
1864, 1867, 1870		Hotten Sl. Dict.
1882		Sydney Sl. Dict. (2 edn) 7: Patent Coat - Inside skirt coat-pocket.