The Collaborative International Dictionary of English v.0.48:

Rejoin \Re*join"\, v. i.
   1. To answer to a reply.
      [1913 Webster]

   2. (Law) To answer, as the defendant to the plaintiff's
      replication.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rejoin \Re*join"\ (r?-join"), v. t. [imp. & p. p. Rejoined
   (-joind"); p. pr. & vb. n. Rejoining.] [F. rejoindre; pref.
   re- re- + joindre to join. See Join, and cf. Rejoinder.]
   1. To join again; to unite after separation.
      [1913 Webster]

   2. To come, or go, again into the presence of; to join the
      company of again.
      [1913 Webster]

            Meet and rejoin me, in the pensive grot. --Pope.
      [1913 Webster]

   3. To state in reply; -- followed by an object clause.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rejoin
    v 1: join again
    2: answer back [syn: retort, come back, repay, return,
       riposte, rejoin]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "rejoin":
   acknowledge, answer, answer back, come back, come back at, come in,
   confute, countercharge, counterclaim, echo, flash back,
   give acknowledgment, give answer, make a rebuttal, react, rebut,
   reecho, refute, reply, respond, retort, return, return answer,
   return for answer, reverberate, riposte, say, say in reply,
   shoot back, surrebut, surrejoin, talk back