Source : Webster's Revised Unabridged Dictionary (1913)

Swallowtail \Swal"low*tail`\, n.
   1. (Carp.) A kind of tenon or tongue used in making joints.
      See {Dovetail}.

   2. (Bot.) A species of willow.

   3. (Fort.) An outwork with converging sides, its head or
      front forming a re["e]ntrant angle; -- so called from its
      form. Called also {priestcap}.

   4. A swallow-tailed coat.

            This Stultz coat, a blue swallowtail, with yellow
            buttons.                              --Thackeray.

   5. An arrow. --Sir W. Scott.

   6. (Zo["o]l.) Any one of numerous species of large and
      handsome butterflies, belonging to Papilio and allied
      genera, in which the posterior border of each hind wing is
      prolongated in the form of a long lobe.

   Note: The black swallowtail, or asterias (see {Papilio}), the
         blue swallowtail, or philenor, the tiger swallowtail,
         or turnus (see {Turnus}), and the zebra swallowtail, or
         ajax (see under {Zebra}) are common American species.
         See also {Troilus}.

Source : WordNet®

swallowtail
     n : a man's full-dress jacket with two long tapering tails at
         the back [syn: {swallow-tailed coat}, {morning coat}]