The Collaborative International Dictionary of English v.0.48:

hee-haw \hee-haw\ v. i.
   to bray in the manner of a donkey.

   Syn: bray.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

hee-haw \hee-haw\ n.
   a loud laugh that sounds like a horse neighing.

   Syn: horselaugh, ha-ha, haw-haw.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

hee-haw
    n 1: a loud laugh that sounds like a horse neighing [syn: hee-
         haw, horselaugh, ha-ha, haw-haw]
    v 1: braying characteristic of donkeys [syn: hee-haw, bray]