The Collaborative International Dictionary of English v.0.48:

Forsooth \For*sooth"\, adv. [AS. fors[=o][eth]; for, prep. +
   s[=o][eth] sooth, truth. See For, prep., and Sooth.]
   In truth; in fact; certainly; very well; -- formerly used as
   an expression of deference or respect, especially to woman;
   now used ironically or contemptuously.
   [1913 Webster]

         A fit man, forsooth, to govern a realm!  --Hayward.
   [1913 Webster]

         Our old English word forsooth has been changed for the
         French madam.                            --Guardian.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Forsooth \For*sooth"\, v. t.
   To address respectfully with the term forsooth. [Obs.]
   [1913 Webster]

         The captain of the "Charles" had forsoothed her, though
         he knew her well enough and she him.     --Pepys.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Forsooth \For*sooth"\, n.
   A person who used forsooth much; a very ceremonious and
   deferential person. [R.]
   [1913 Webster]

         You sip so like a forsooth of the city.  --B. Jonson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

forsooth
    adv 1: an archaic word originally meaning `in truth' but now
           usually used to express disbelief

Moby Thesaurus II by Grady Ward, 1.0:

42 Moby Thesaurus words for "forsooth":
   absolutely, actually, and no mistake, assuredly, at all events,
   at any rate, by all means, certainly, clearly, decidedly,
   decisively, definitely, distinctly, factually, for a certainty,
   for a fact, for certain, for sure, historically, in fact,
   in reality, in truth, in very sooth, indeed, indeedy,
   most assuredly, most certainly, nothing else but, of a truth,
   of course, positively, really, really-truly, surely,
   to a certainty, truly, undoubtedly, unequivocally, unmistakably,
   verily, veritably, with truth