स्वाङ्गाच्चोपसर्जनादसंयोगोपधात्

And the feminine affix ङीष् comes optionally after what ends with the name of a part of the body when the word is a subordinate member in compound and has not a conjunct for its penultimate letter (i.e. the final अ is not preceded by a double consonant).
The Anuvritti of the words बहुव्रीहेः, क्तान्तादन्तोदात्तात् does not extend to this aphorism. But the anuvritti of the word वा 'optionally' does extend to this Sutra. Thus चन्द्रमुखी or चन्द्रमुखा, अतिकेशी or अतिकेशा ॥ The word अतिकेशा is a Tatpurusha compound meaning अतिक्रान्ता केशान्, and केश is upasarjana by 1.2.44.

Why do we say "what ends with the name of a part of the body"? Observe बहुयवा 'having much barley'. Why do we say "subordinate in a compound"? Witness अशिखा "without crest" (where the शिखा is not subordinate or epithetical). Why do we say "not having a conjunct for its penultimate letter"? Witness सुगुल्फा, सुपार्श्वा ॥

Vart :- This rule applies to a compound ending with the word अङ्ग, गात्र and कण्ठ, though in all these, the penultimate is a double consonant. Thus मृद्वङ्गी or मृद्वङ्गा; सुगात्री or सुगात्रा, स्निग्धकण्ठी or स्निग्धकण्ठा ॥

The word स्वांग has a technical meaning as given in the following verse:-

अद्रवं मूर्तिमत्स्वाङ्गं प्राणिस्थमविकारजम् ।
अतत्स्थं तत्रदृष्टं चेत् तेन चेत् तत्तथायुतम् ॥

A word denoting a thing which not being a fluid, is capable of being perceived by the senses because of its having a form, is svanga; it must exist in a living being, but not produced by a change from the natural state; or though found elsewhere actually, had previously been known as existing in only a living being, or is found to have actually the same relation to the being it is in, as a similar thing has to a living being.