swallowtail

 Swallowtail butterfly on Wikipedia

 Swallowtail (flag) on Wikipedia

 Tailcoat on Wikipedia

From swallow +‎ tail.

• (UK) IPA^(key): /ˈswɒl.əʊˌteɪl/

swallowtail (plural swallowtails)

1. The forked tail of a swallow.
2. Anything, such as a burgee, of a similar forked shape.
3. A type of tailcoat with two long tapering tails.
   • 1906, Stanley J[ohn] Weyman, chapter I, in Chippinge Borough, New York, N.Y.: McClure, Phillips & Co., →OCLC, page 01:

     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.

4. Any of various butterflies of the family Papilionidae, many of which have a long extension to each hindwing.
   • 2020, Paul Mendez, Rainbow Milk, Dialogue Books (2021), page 8:

     Claudette visit me […] and the biggest, most beautiful black-and-yellow Jamaican swallowtail butterfly follow behind she, like it give us it little blessing.