Green’s Dictionary of Slang

a long gaberdine macintosh, reaching to the ankles.

1855		Household Words 14 Apr. 254/1: Toryism in an all-round collar, a Noah’s Ark coat, Sydenham trousers, and a downy moustache, is simply a monster.
1860		Bennett & Brough Shawdow and Substance 104: Tops, marbles, kites, suckers, squirts, crossbows, and all similar matters, are looked upon by Young Rapid much as a Bond Street exquisite, in a Noah’s ark coat and pegtop trousers, would contemplate pigtails, Hessian boots, and hair powder.
1874		Leeds Times 27 June 6/2: I saw him once in a Noah’s Ark coat —a long, straight-skirted gaberdine, reaching well-nigh to the heels.
1889		S. & M. Bancroft Mr. and Mrs. Bancroft 62: The long claret-coloured Noah’s Ark coat which he wore on the first night he borrowed from Dion Boucicault.
1903		Int’l Studio 20 248/2: Young painters who, having lately seen Whistler, will come up to one with all the master’s little airs and graces, his straight-brimmed hat and long Noah’s Ark coat, his parrot scream, and his ‘Ha, ha! Amazing’.