The Collaborative International Dictionary of English v.0.48:

hairball \hair"ball`\, hair ball \hair" ball`\(h[^a]r"b[e^]l`),
   n.
   a compact mass of hair that forms in the stomach of animals
   as a result of licking fur; as, the cat coughed up a hairball
   right on the new rug. [WordNet sense 1 + 2]
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

hairball
    n 1: a compact mass of hair that forms in the alimentary canal
         (especially in the stomach of animals as a result of
         licking fur) [syn: hairball, hair ball, trichobezoar]

The Jargon File (version 4.4.7, 29 Dec 2003):

hairball
 n.

    1. [Fidonet] A large batch of messages that a store-and-forward network is
    failing to forward when it should. Often used in the phrase ?Fido coughed
    up a hairball today?, meaning that the stuck messages have just come
    unstuck, producing a flood of mail where there had previously been drought.

    2. An unmanageably huge mass of source code. ?JWZ thought the Mozilla
    effort bogged down because the code was a huge hairball.?

    3. Any large amount of garbage coming out suddenly. ?Sendmail is coughing
    up a hairball, so expect some slowness accessing the Internet.?