The Collaborative International Dictionary of English v.0.48:

Ha \Ha\ (h[aum]), interj. [AS.]
   An exclamation denoting surprise, joy, or grief. Both as
   uttered and as written, it expresses a great variety of
   emotions, determined by the tone or the context. When
   repeated, ha, ha, it is an expression of laughter,
   satisfaction, or triumph, sometimes of derisive laughter; or
   sometimes it is equivalent to "Well, it is so."
   [1913 Webster]

         Ha-has, and inarticulate hootings of satirical rebuke.
                                                  --Carlyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

HA
    n 1: (astronomy) the angular distance of a celestial point
         measured westward along the celestial equator from the
         zenith crossing; the right ascension for an observer at a
         particular location and time of day [syn: hour angle,
         HA]