The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Carp.) United by dovetailing; dovetailed.
      [1913 Webster]

   Swallow-tailed duck (Zool.), the old squaw.

   Swallow-tailed gull (Zool.), an Arctic gull (Xema
      furcata), which has a deeply forked tail.

   Swallow-tailed hawk or Swallow-tailed kite (Zool.), the
      fork-tailed kite.

   Swallow-tailed moth (Zool.), a European moth (Urapteryx
      sambucaria) having tail-like lobes on the hind wings.
      [1913 Webster]

WordNet (r) 3.0 (2006):

swallow-tailed
    adj 1: (especially of butterflies and birds) having a forked
           tail like that of a swallow