Shloka 27
Adhyāya 166: Kṛtaghna-doṣa (कृतघ्नदोषः) — the fault of ingratitude and the limits of expiation
उदपानोदके ग्रामे ब्राह्मणो वृषलीपति: । उषित्वा द्वादश समा: शूद्रकर्मेव गच्छति
udapānodake grāme brāhmaṇo vṛṣalīpatiḥ | uṣitvā dvādaśa samāḥ śūdrakarmāiva gacchati ||
ビーシュマは言った。「一つの井戸の水を皆が飲む村において、婆羅門が首陀羅（Śūdra）の女を妻とし、そこに十二年住み続けるなら、その者は首陀羅の行いと生業に従う者と見なされる。」この偈は、社会的な身分が長期の交わり、婚姻、そして生計によって形づくられることを示し、非婆羅門的な生活様式に久しく浸れば、認められるヴァルナ（varṇa）の地位と義務もまたそれに応じて移り変わると警告する。
भीष्म उवाच
Educational Q&A
The verse teaches that varṇa-identity and eligibility for particular dharmas are not treated as merely nominal; sustained association, marriage alliance, and long-term residence within a community—together with adopting its customary occupations—can lead society to regard a person as having shifted into that community’s mode of life and duties.
In the Śānti Parva’s dharma-instruction, Bhishma is laying down a rule-like observation about social status: a Brahmin who marries a Śūdra woman and lives for twelve years in a village characterized by common use of a single well is said to ‘go to’ (i.e., be classed as) one following Śūdra conduct/occupation.