Adhyāya 166: Kṛtaghna-doṣa (कृतघ्नदोषः) — the fault of ingratitude and the limits of expiation
सुरापान ब्रह्म॒हत्या गुरुतल्पमथापि वा । अनिर्देश्यानि मन्यन्ते प्राणान््तमिति धारणा
surāpāna-brahmahatyā guru-talpam athāpi vā | anirdeśyāni manyante prāṇāntam iti dhāraṇā ||
Bhishma said: Drinking intoxicants, the killing of a brāhmaṇa, and violating the teacher’s bed (i.e., sexual transgression with the guru’s wife)—these are regarded as sins for which no ordinary expiation can be prescribed. The learned hold that only the ending of one’s life is the expiation for such acts.
भीष्म उवाच
Bhīṣma classifies certain acts—liquor-drinking, brāhmaṇa-slaying, and violating the guru’s bed—as mahāpātakas so grave that ordinary ritual expiations are considered inadequate; some authorities therefore regard death (self-ending) as the only expiation.
In the Śānti Parva’s dharma-instruction, Bhīṣma is advising Yudhiṣṭhira on moral law and penance, explaining how traditional authorities evaluate the heaviest transgressions and what they deem to be their (non-)expiability.