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