Nirapāya (from nir- = without, apāya = fall/defect/obstacle) means 'unfailing,' 'infallible,' or 'without possibility of decline.' In Śrī Vaiṣṇava pāribhāṣika usage, it is most characteristically applied to prapatti as nirapāya upāya — the means that cannot fail. This contrasts prapatti with karma-yoga and jñāna-yoga, which are termed sāpāya (with possible failure), being dependent on the practitioner's sustained effort, physical capacity, and freedom from disqualifying karma.
Piḷḷai Lokācārya's Mumukṣuppati explains why prapatti is nirapāya: unlike other upāyas that are performed (sādhana-bhūta) by the aspirant, prapatti positions Bhagavān Himself as the upāya. Since Bhagavān's svātantrya (absolute sovereignty) and kāruṇya (compassion) are themselves unfailing, the means that relies on them cannot fail. The prapanna is like a child held in the arms of a powerful parent — the child's own strength is irrelevant to its safety. This is why Maṇavāḷa Māmunigaḷ calls it nirapāya rakṣaka — the infallible protector.
The concept of nirapāya has important practical implications for the prapanna's psychology. It removes the anxiety of self-monitoring that characterises sādhana-based paths. The prapanna is not worried whether his surrender was 'enough' or whether a later lapse cancels it — because the efficacy is entirely on the side of Bhagavān, whose commitment is nirapāya. The term is thus both a description of prapatti's structure and a source of abhaya (fearlessness) for the surrendered soul.