Aparihāna-dhamma: 'incapable of relapse', or 'of falling away', namely, with regard to deliverance from some or all mental chains of existence see: samyojana. Thus all Noble Disciples are called, i.e. all those who have attained any of the 4 noble paths to Nobility see: ariya-puggala. With regard to the absorption jhanas, anyone is called 'unrelapsable' who has attained full mastery over the absorptions.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.