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]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "forthwith":
   PDQ, abruptly, all at once, all together, apace, at a blow,
   at a stroke, at once, at one blow, at one jump, at one stroke,
   at one swoop, at one time, away, decisively, directly,
   expeditiously, immediately, in a hurry, in no time, instanter,
   instantly, now, on the instant, on the spot, per saltum,
   pretty damned quick, promptly, pronto, quickly, right away,
   right now, right off, simultaneously, smartly, speedily,
   straightaway, straightway, subito, sudden, suddenly, summarily,
   swiftly, then and there, this minute, this very minute, uno saltu,
   with all speed, without delay, without further delay



Bouvier's Law Dictionary, Revised 6th Ed (1856):

FORTHWITH. When a thing is to be done forthwith, it seems that it must be
performed as soon as by reasonable exertion, confined to that object, it may
be done. This is the import of the term; it varies, of course, with every
particular case. 4 Tyr. 837; Styles' Register, 452, 3.