Kuppa-dhamma: 'liable to perturbation', is one who has not yet attained full mastery over the absorptions. In Pug. 3 it is said: What person is liable to perturbation? Such a person gains the attainments of the fine-material and immaterial sphere see: avacara But he does not gain them at his wish, nor without toil and exertion; and not at his wish as regards place, object and duration, does he enter them or arise from them. Thus it is well possible that in case of such a person, through negligence, the attainments will become perturbed. This person is liable to perturbation.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.