शॄवन्द्योरारुः

Operational rule vidhi

The affix आरु comes, in the sense of 'agent having such a habit etc.' after the verbs शॄ 'to to injure' and वन्द् 'to praise, to salute'.

Word by word (padaccheda)

Target शॄ- वन्द्योः ṣaṣṭhī · dvivacanaSubstitute / term आरुः 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

शॄ-वन्द्योः आरुः

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)

शॄ- वन्द्योः → आरुः

Replace śṝ-vandyoḥ with āruḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

The affix आरु comes, in the sense of 'agent having such a habit etc.' after the verbs शॄ 'to to injure' and वन्द् 'to praise, to salute'.
As शरारुः 'noxious, a mischievous animal'; वन्दारुः 'praising, a bard'.

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