Dear Martin Gansten
This probably means
Vyakarana Mahabhasya which is written by Patanjali.
He is believed to be an incarnation of Shesharaj who is a bed of Lord Vishnu. So Ahi means Snake. Gavi means his speech or Vanee. In Sanskrit gow means cow and also speech. Here in this context the meaning is to be considered as speech and derivative of the same would lead to this new word.
This is used by famous commentator Mallinatha in his Raghuvamsa etc at the commencement of the same.