यूनश्च कुत्सायाम्

Definition saṃjñā युवसंज्ञा

The युवन् is optionally spoken of as गोत्र when contemptuous reference to him is intended.

Word by word (padaccheda)

Of यूनः ṣaṣṭhī · ekavacanaIndeclinable च avyayaIn / when कुत्सायाम् saptamī · ekavacana

Of Ṣaṣṭhī (6th case): "of X". In this kind of sūtra it marks a relation, not a substitution.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".
In / when Saptamī (7th case): "in X" or "when X", a domain or condition.

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Full reading

यूनः च कुत्सायाम्

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

Commentary

The युवन् is optionally spoken of as गोत्र when contemptuous reference to him is intended.
Thus गार्ग्यो जाल्मः or गार्ग्यायणो जाल्मः ॥ When contempt is not intended, the only form is गार्ग्यायणः ॥ This is also a Vartika and not a sutra.

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