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.”