Wiktionary
interj. 1 (context often repeated several times English) The sound of a cat coughing up a hairball. 2 Also used of similar noises, particularly one made to show disgust or disapproval. vb. 1 (context intransitive English) To make a 'gack' noise. 2 (context transitive English) To expectorate a hairball.
Wikipedia
Gack is a village in the administrative district of Gmina Drawno, within Choszczno County, West Pomeranian Voivodeship, in north-western Poland. It lies approximately south of Drawno, east of Choszczno, and east of the regional capital Szczecin.
Before 1945 the area was part of Germany. For the history of the region, see History of Pomerania.
Usage examples of "gack".
He didn't scoff often, so he wasn't good at it, and it appeared to Gabe and Val that he might be gacking up a hair ball.