'unshakable', is 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 immaterial sphere (rūpāvacara-arūpāvacara); and he gains them at his wish, without toil and exertion; and according to his wish, as regards place, object and duration, enters them or arises from them, then it is impossible that in such a person the attainments may become shaken through negligence. This person is unshakable."