Adhyāya 166: Kṛtaghna-doṣa (कृतघ्नदोषः) — the fault of ingratitude and the limits of expiation
संवत्सरेण पतति पतितेन सहाचरन् | याजनाध्यापनाद यौनान्न तु यानासनाशनात्
saṃvatsareṇa patati patitena sahācaran | yājanādhyāpanād yaunān na tu yānāsanāśanāt ||
Bhishma said: “A man becomes fallen within a year by keeping company with one who is fallen; likewise by officiating at his sacrifices and by teaching him. But by entering into marital alliance with his line, or by sharing the same conveyance, seat, or food with him, the fall is not delayed for a year—it is immediate.”
भीष्म उवाच
Bhishma distinguishes degrees of moral/ritual contamination: some forms of association with a ‘fallen’ person lead to degradation over time (general companionship, priestly service, teaching), while intimate boundary-crossing acts—marital/sexual alliance and sharing conveyance, seat, or food—are treated as causing immediate fall.
In the Shanti Parva’s dharma-instruction, Bhishma is laying down norms of conduct and social boundaries, explaining to the listener how different interactions with a patita are evaluated and what consequences they are said to bring.