The Collaborative International Dictionary of English v.0.48:

Aha \A*ha"\, interj. [Ah, interj. + ha.]
   An exclamation expressing, by different intonations, triumph,
   mixed with derision or irony, or simple surprise.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Aha \A*ha"\, n.
   A sunk fence. See Ha-ha. --Mason.
   [1913 Webster]

Easton's 1897 Bible Dictionary:

Aha!
   an exclamation of ridicule (Ps. 35:21; 40:15; 70:3). In Isa.
   44:16 it signifies joyful surprise, as also in Job 39:25, R.V.