Green’s Dictionary of Slang

of a coat, having a long tail.

1826		C.M. Westmacott Eng. Spy II 170: We must start that long-tailed gib of yours for a nice little square mizen, just enough to cover your beam.
1849		J.P. Townsend Rambles in New South Wales 168: The phrase ‘long-tailed’ is an allusion to the emigrant’s carefully-kept church-going coat.
1872		‘Mark Twain’ Innocents at Home 345: A stranger with a long-tailed coat came in.
1904		Sydney Sportsman (Surry Hills, NSW) 9 Nov. 3/5: [He] took off his long-tailed banger [and] hung it on a peg near the door.