Manava Dharmashastra 4.82

4.82

saṃhatābhyāṃ saṃśliṣṭābhyāmitaretarasaṃsṛṣṭābhyāṃ yugapadbhvābhyāṃ pratiṣedhaḥ |
pāṇibhyāmiti bāhū saṃhatau niṣedhati |
ātmana iti na parasya | ataścānyena saṃhatābhyāṃ kaṇḍūyato na doṣaḥ |
śirograhaṇātpṛṣṭhādāvadoṣaḥ |
na spṛśeccaita śiraḥ | hastenātmano'nyena vā'vayaveneti kecit | tanna, pāṇibhyāmiti prakṛtatvāt |
na ca snāyācchirasā vinā | nityanaimittikayoḥ snānayorayaṃ vidhiḥ |
‘‘nanu svinnasya laukike snāne kuta etat’’ |
snānavidhinaikavākyatvāt |
‘‘vihitasnānāpekṣā pratyāsattyā yuktimatī | loke tu vidherabhāvādaprāptiḥ’’ |
snātiścāyaṃ sarvāṅgasambandhini salilagomūtrādi prakṣālane vartate, śirovarjite ca | tatra caṇḍālādisparśane śirovarjitamapi yadṛcchāprasaktaṃ nivāryate—na ca snāyādvinā tataḥ | asti ca laukikamaśiraskamapi snānam | yena śiraḥsnānaṃ ‘‘śiraḥ snātastu taileneti’’ |

He shall not scratch his own head with both hands joined together; he shall not touch it while unclean; and he shall not bathe without it.—(82).

Medhātithi’s commentary (manubhāṣya):
‘Joined together’—joined to one another. This forbids the scratching of the head with both hands at the same time.

Hands; this forbids the use of the two arms joined together.

‘His own’— not that of others; hence there would be nothing wrong in having one’s head scratched with the two hands of another person.

Since the head has been specified, there would be nothing wrong in scratching the back and other parts of the body.

‘He shall not touch it’—his own head, with his own hand,—or with any other part of the body, as some people have explained. But this is not right, as it is the hands that are being spoken of in the text.

‘He shall not bathe without it’—i.e., without the head. This rule applies to all kinds of bathing—the daily obligatory one as well as the occasional one.

“Why should this rule be observed in the case of ordinary bathing, done by a person who has perspired (and only wishes to clean the perspiration)?”

That it should be so follows from the fact that the present rule is meant to be taken along with the rule laying down bathing.

For connecting this rule with the bathing that is directly enjoined,—there may be some reason. But, so far as the ordinary bathing is concerned, since there is no injuction regarding it, there can be no ground for observing the present rule in connection with it.”

Well, as a matter of fact, the root‘to bathe’ denotes the act of washing with water, cow’s urine and such things—the whole body or the rest of the body, barring the head. And, since people might leave off the head, when bathing on having touched a Cāṇḍāla, or some such unclean thing,—the text forbids this by the rule—‘he shall not bathe without the head.’ Ordinary bathing, without washing the head, is of course possible; in view of which we have such assertions as—‘having bathed his head, etc., etc.,’—(82)

This verse is quoted in Aparārka (p. 183), which explains ‘tataḥ’ as standing for the head.

Comparative notes by various authors
Viṣṇu (68.38.)—‘He shall not touch his head (while unclean).’

Viṣṇu (71.53).—‘With hands joined together, he shall not scratch his head or his belly.’

Mahābhārata (13.101-69).—[Same as Manu, the second line reading as —‘ Nacābhīkṣṇam śiraḥ snāyāt tathāsyāyurna ṛṣyate ].’