Source : Webster's Revised Unabridged Dictionary (1913)

Swallow-tailed \Swal"low-tailed`\, a.
   1. Having a tail like that of a swallow; hence, like a
      swallow's tail in form; having narrow and tapering or
      pointed skirts; as, a swallow-tailed coat.

   2. (Carp.) United by dovetailing; dovetailed.

   {Swallow-tailed duck} (Zo["o]l.), the old squaw.

   {Swallow-tailed gull} (Zo["o]l.), an Arctic gull ({Xema
      furcata}), which has a deeply forked tail.

   {Swallow-tailed} {hawk or kite} (Zo["o]l.), the fork-tailed
      kite.

   {Swallow-tailed moth} (Zo["o]l.), a European moth ({Urapteryx
      sambucaria}) having tail-like lobes on the hind wings.

Swallow-tailed \Swal"low-tailed`\, a.
   1. Having a tail like that of a swallow; hence, like a
      swallow's tail in form; having narrow and tapering or
      pointed skirts; as, a swallow-tailed coat.

   2. (Carp.) United by dovetailing; dovetailed.

   {Swallow-tailed duck} (Zo["o]l.), the old squaw.

   {Swallow-tailed gull} (Zo["o]l.), an Arctic gull ({Xema
      furcata}), which has a deeply forked tail.

   {Swallow-tailed} {hawk or kite} (Zo["o]l.), the fork-tailed
      kite.

   {Swallow-tailed moth} (Zo["o]l.), a European moth ({Urapteryx
      sambucaria}) having tail-like lobes on the hind wings.

Source : WordNet®

swallow-tailed
     adj : (especially of butterflies and birds) having a forked tail
           like that of a swallow