ऊरूत्तरपदादौपम्ये

Operational rule vidhi

When the feminine affix ऊङ् comes after a stem ending in ऊरु when comparison with something (expressed by the first term) is meant.

Word by word (padaccheda)

Left context ऊरू- उत्तरपदात् pañcamī · ekavacanaRight context औपम्ये 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.
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").

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Full reading

ऊरू-उत्तरपदात् औपम्ये

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

Commentary

When the feminine affix ऊङ् comes after a stem ending in ऊरु when comparison with something (expressed by the first term) is meant.
Thus कदलास्तम्भोरूः 'a female with thighs like plantain-stem'. नागनासोरूः 'a female with thighs like the nose of an elephant'; करभोरूः 'female with thighs like the ulnar or tapering fleshy side of the hand'.

Why do we say "when expressing an object of comparison"? Witness वृत्तोरूः स्त्री 'a woman with round thighs'. The word करेणुकरोरु "thigh like the proboscis (कर)of an elephant (करेणु)" is a poetic irregularity in the following विचकरे च करेणुकरोरुभिः ॥

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