Definition of "swallowtail"
A type of tailcoat with two long tapering tails.
Quotations
It was April 22, 1831, and a young man was walking down Whitehall in the direction of Parliament Street. He wore shepherd's plaid trousers and the swallow-tail coat of the day, with a figured muslin cravat wound about his wide-spread collar.
1906, Stanley J[ohn] Weyman, chapter I, in Chippinge Borough, New York, N.Y.: McClure, Phillips & Co., page 01