lit. 'That which cannot or should not be thought, the unthinkable, incomprehensible, impenetrable, that which transcends the limits of thinking and over which therefore one should not ponder.
These 4 unthinkables are:
the sphere of a Buddha (buddha-visaya),
of the meditative absorptions (jhāna-visaya),
of karma-result (kamma-vipāka), and
brooding over the world (loka-cintā), especially over an absolute first beginning of it
"Therefore, o monks, do not brood over the world as to whether it is eternal or temporal, limited or endless .... Such brooding, O monks, is senseless, has nothing to do with genuine pure conduct (s. ādibrahmacariyaka-sīla), does not lead to aversion, detachment, extinction, nor to peace, to full comprehension, enlightenment and Nibbāna, etc." (S.56.41).