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.