स्वपितृषोर्नजिङ्

Operational rule vidhi

The affix नजिङ् comes, in the sense of 'agent having such a habit etc.' after the verbs स्वप् 'to sleep' and तृष् 'to be thirsty'.

Word by word (padaccheda)

Target स्वपि- तृषोः ṣaṣṭhī · dvivacanaSubstitute / term नजिङ् prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

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Full reading

स्वपि-तृषोः नजिङ्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

स्वपि- तृषोः → नजिङ्

Replace svapi-tṛṣoḥ with najiṅ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The affix नजिङ् comes, in the sense of 'agent having such a habit etc.' after the verbs स्वप् 'to sleep' and तृष् 'to be thirsty'.
The affix नजिङ् comes, in the sense of 'the agents having such a habit &c', after the verbs स्वप् 'to sleep' and तृष 'to be thirsty'.

As स्वप् + नजिङ् = स्वप्नज् Ist. sing. स्वप्नक् 'sleeping, drowsy'; तृष्णज् Ist. sing. तृष्णक् 'covetous, thirsty'.

Vart:- So also after the verb धृष् 'to insult, be bold'; as, धृष्णज् Ist. sing. धृष्णक् 'bold, impudent'.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.