वाकिनादीनां कुक् च

Operational rule vidhi

The augment क् (कुक्) is added to the words वाकिन etc. when the Patronymic affix फिन् follows.

Word by word (padaccheda)

Target वाकिन- आदीनाम् ṣaṣṭhī · bahuvacanaSubstitute / term कुक् prathamā · ekavacanaIndeclinable च avyaya

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.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

वाकिन-आदीनाम् कुक् च

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)

वाकिन- आदीनाम् → कुक्

Replace vākina-ādīnām with kuk. Inherited words (anuvṛtti) may add further conditions.

Commentary

The augment क् (कुक्) is added to the words वाकिन etc. when the Patronymic affix फिन् follows.
This वाकिनकायनिः, गोरधकायनिः ॥ This debars इञ् &c. But if the anuvritti of उदीचां is read into this aphorism, then it becomes an optional rule. In that case we have in the alternative :- वाकिनिः, गोरधिः ॥

1 वाकिन, 2 गौधेर (गारेध K.) 3 कार्कष (कार्कट्य K,) 4 काक, 5 लङ्का, 6 चर्मिवर्मिणोर्नलोपश्च ॥

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