Akuppa-dhamma: The unshakeable state; is that of one who has attained full mastery over the absorptions jhāna. In Pug. 4 it is said: What person is unshakable? If a person gains the meditative attainments of the fine-material and formless sphere rūpāvacara-arūpāvacara; and he gains them at his wish, without any problem or strain with free choice of place, object and duration, and enters them and emerges from them effortlessly, then it is impossible that in such a person the attainments may become shaken through negligence. This person is unshakeable.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.