8.3.86
अभिनिसः स्तनः शब्दसंज्ञायाम्
Operational rule vidhi
ष् is optionally substituted for the स् of स्तन् after the double उपसर्ग (Preposition) अभि निस् when the word so formed is the name of a particular letter (विसर्ग).
Word by word (padaccheda)
Left context अभि- निसः pañcamī · ekavacanaTarget स्तनः ṣaṣṭhī · ekavacanaRight context शब्दसंज्ञायाम् saptamī · ekavacana
- Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
- Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
- Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Full reading
अभि-
Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.
Commentary
ष् is optionally substituted for the स् of स्तन् after the double उपसर्ग (Preposition) अभि निस् when the word so formed is the name of a particular letter (विसर्ग).
As अभिनिष्टानो वर्णः, अभिनिष्टानो विसर्जनीयः or अभिनिस्तानो वर्णः or विसर्जनीयः ॥ The compound preposition अभि-निस् causes this change, and not any one of them separately. Thus Apastamba :- द्व्यक्षरं चतुरक्षरं वा नामपूर्वमाख्यातोत्तरं दीर्घभिनिष्टान्तं, घोषवदाद्यन्तरन्तस्थम् "a name (nama) should be such that it should consist of two-syllables or four-syllables, that its first portion should be a noun (nama-purva), and its second portion a verb (akhyata), that it should end in a long vowel (dirgha) or a visarga (abhinishtana), that the first letter of such a noun should be a ghosha or sonant letter (ghosha-vad-adi), and a semi vowel (antastha) should be in the body of it." Thus the names द्रविणोदा, वरिवोदा fulfill these conditions and are good names.
Why do we say "when it is the name of a particular letter."? Observe अभिनिस्तनति मृदङ्गः ॥ The anuvritti of समासे ceases from this place.
Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.