The Collaborative International Dictionary of English v.0.48:

Priestcap \Priest"cap`\, n. (Fort.)
   A form of redan, so named from its shape; -- called also
   swallowtail.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Swallowtail \Swal"low*tail`\, n.
   1. (Carp.) A kind of tenon or tongue used in making joints.
      See Dovetail.
      [1913 Webster]

   2. (Bot.) A species of willow.
      [1913 Webster]

   3. (Fort.) An outwork with converging sides, its head or
      front forming a reentrant angle; -- so called from its
      form. Called also priestcap.
      [1913 Webster]

   4. A swallow-tailed coat.
      [1913 Webster]

            This Stultz coat, a blue swallowtail, with yellow
            buttons.                              --Thackeray.
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   5. An arrow. --Sir W. Scott.
      [1913 Webster]

   6. (Zool.) 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.
      [1913 Webster]

   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.
         [1913 Webster]