Green’s Dictionary of Slang

a swallow-tailed coat; the tail of such a coat; thus swallow-tailed adj., dressed in such a coat, or, of a garment, having a swallow tail; also attrib.

1834		Marryat Peter Simple (1911) 393: May I be so bold as to ax, Captain O’Brien, whether I must wear one of them long tog, swallow-tailed coats?
1842		letter R.B. Hayes in Williams Diary and Letters (1922) I 98: The hero [...] appeared as all young men do when arrayed for the first time in a whole coat with brass buttons and swallow-tail.
1851		Melville Moby Dick (1907) 439: A long-tailed coat ought always to be worn in all storms afloat [...] No more monkey-jackets and tarpaulins for me; I must mount a swallow-tail and drive down a beaver.
1866		Night Side of N.Y. 7: A very respectable individual with carefully shaved whiskers, black suit, swallow tail cut, white cravat and slippers.
1875		M.D. Landon Eli Perkins 137: Gentlemen in ‘swallow tails’.
1885		Bulletin (Sydney) 28 Feb. 11/1: Yes, dearest, I think you had better put in my swallow-tail, &c., as I have no doubt we’ll be invited out a good deal.
1890		Bulletin (Sydney) 5 July 9/3: As Levien, M.P., puts it, ‘broken-down swells, mere swallow-tailed men from England,’ are rushing the N.S.W. civil service.
1908		J. Guinan Soggarth Aroon 148: He had on his Sunday suit, a cutaway, or ‘swallow-tail’.
1914		S. Lewis Our Mr Wrenn (1936) 248: I wouldn’t know when to wear one of them things, and when a swallow-tail – if I had one.
1918		Dagger (London) Dec. I 4/2: He had on when last seen a pair of swallow-tailed trousers.
1925		S. Lewis Arrowsmith 459: Do I put on the soup-and-fish? Oh, I got ’em – swallow-tail and the whole darn’ outfit!
1935		Z.N. Hurston Mules and Men (1995) 84: John goes in and dressed up in Ole Massa’s swaller-tail clothes.
a.1986		‘Old Zebra Dun’ in G. Logsdon Whorehouse Bells Were Ringing (1995) 82: You could tell he was a feller not accustomed to the trail, / With his ’gators and his leggin’s and his fancy swaller tail.