The Collaborative International Dictionary of English v.0.48:

Ergo \Er"go\, conj. or adv. [L.]
   Therefore; consequently; -- often used in a jocular way.
   --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ergo
    adv 1: (used as a sentence connector) therefore or consequently

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "ergo":
   accordingly, because of that, because of this, consequently,
   for that, for that cause, for that reason, for this cause,
   for this reason, for which reason, hence, hereat, on that account,
   on that ground, on this account, propter hoc, so, then, thence,
   thereat, therefor, therefore, thereupon, thus, thusly, thuswise,
   whence, wherefore, wherefrom