The Collaborative International Dictionary of English v.0.48:

Forthwith \Forth`with"\ (? or ?; see With), adv.
   1. Immediately; without delay; directly.
      [1913 Webster]

            Immediately there fell from his eyes as it had been
            scales; and he received sight forthwith. --Acts ix.
                                                  18.
      [1913 Webster]

   2. (Law) As soon as the thing required may be done by
      reasonable exertion confined to that object. --Bouvier.
      [1913 Webster]

WordNet (r) 3.0 (2006):

forthwith
    adv 1: without delay or hesitation; with no time intervening;
           "he answered immediately"; "found an answer
           straightaway"; "an official accused of dishonesty should
           be suspended forthwith"; "Come here now!" [syn:
           immediately, instantly, straightaway, straight
           off, directly, now, right away, at once,
           forthwith, like a shot]