Yāska's Nirukta 188.1.5

188.1.5

"mithunau kasmāt ?" "minotiḥ" tāvadatra "śrayatikarmā" mithunaśabde pūrvapadaṃ 'mi'-iti | 'thu'-iti nāmakaraṇaḥ pratyayaḥ, "thakāro vā" nāmakaraṇo vikalpena | "nayatiḥ paraḥ" uttarapadaṃ, "vanirvā" vikalpena | yadā tāvat 'thu'-iti nāmakaraṇo nayatiśca paraḥ, tadā dhātoḥ pūrvatvāt pratyayasya ca paratvāt 'mi-na-thuḥ'--iti prāpte rūḍherbalīyastvāt madhyāntaviparyayeṇa mithunāvityuktam | atha punaryadā thakāro nāmakaraṇaḥ, tadā vaniruttaraḥ, tadā vakārasya samprasāraṇam, samprasāraṇaparapūrvatvaṃ madhyāntaviparyayaḥ | thakāraṃ ca yaḥ svaraḥ, tasya lopaḥ, thakāraśca ukāramupasaṃkrāmet | evaṃ mithunāviti sidhyati || atha ko'rthaḥ ? "samāśritau" "anyonyaṃ" prati ātmānaṃ "nayataḥ" kālaṃ vā samāśritau bhavataḥ parasparaṃ sambhaktau | "manuṣyamithunau api etasmāt eva" | athavā methatermanuṣyamithunau syātām, tau hi karmārtha "methantau" iva paraspareṇa kālaṃ nayataḥ | 7 | 29 | "atha" punaḥ "enam" ādityam "etayā" uttarayarcā etasminneva sūkte "agnīkṛtya" hotṛtvena "stauti" hotṛtvaṃ hyagneḥ karma, na sūryasya | tasmādidamagnivaiśvānarīyaṃ sūktam ||