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,जिस गाँवमें एक ही कुएँका पानी सब लोग पीते हैं, वहाँ बारह वर्षोतक निवास करनेसे तथा शूद्रजातिकी स्त्रीके साथ विवाह कर लेनेसे ब्राह्मण भी शूद्र हो जाता है
udapānodake grāme brāhmaṇo vṛṣalīpatiḥ | uṣitvā dvādaśa samāḥ śūdrakarmāiva gacchati ||
毗湿摩说：在一村之中，众人同饮一井之水；若婆罗门娶首陀罗女子为妻，并在彼处居住十二年，便会被视为遵行首陀罗之行与业者。此偈以为社会身份由长久交往、婚姻与生计所塑成，并警示：长期沉浸于非婆罗门的生活方式，终将引致其被承认的瓦尔那（varṇa）地位与职责随之转移。
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.