from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. The sound of a cat coughing up a hairball.
- interj. Also used of similar noises, particularly one made to show disgust or disapproval.
- v. To make a 'gack' noise.
- v. To expectorate a hairball.
Apparently from the sound made; believed to have first appeared in comic strips. (Wiktionary)