From The Dhamma Encyclopedia
Acinteyya: lit. 'That which cannot not be thought of', the unthinkable, unimaginable, inconceivable, incomprehensible, impenetrable, that which transcends the limits of thinking and over which therefore one should not speculate. The 4 unthinkables are: the potential range of a Buddha buddha-visaya, the potential range of the meditative absorptions jhāna-visaya, the potential range of kammic-result kamma-vipāka, and speculation over the world loka-cintā, especially over an absolute first beginning of it , and whether it is infinite in space and time see: A. IV, 77.
Therefore, o Bhikkhus, do not speculate over the world as to whether it is eternal or temporal, limited or endless. Such speculation, O Bhikkhus, is senseless, has nothing to do with genuine pure conduct see: ādibrahmacariyaka-sīla, does neither lead to aversion, detachment, ceasing, nor to peace, not to full comprehension, not to enlightenment or Nibbana. S.LVI, 41.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.