The affix ṭhak comes in the sense of 'voracious' after the word udara,
The translation given above is according to Dr. Bohtlingk. According to the Kasika, the anuvritti of प्रसित is also read into this sutra. The meaning then would be thak comes after the word udara in the seventh case in construction, in the sense of 'who is devoted therein', when the word so formed means \voracious\. A person who is very much distressed by hunger is so called; उदरेऽविजिगीषुर्भण्यते ॥ Thus उदरे प्रसितः = औदरिकः (i.e. आद्यूनः).
Why do we say when it means 'voracious'? Observe उदरकः 'abdominal'.,