acinteyya: 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 kamma-result (kamma-vipāka), and brooding over the world (loka-cintā), especially over an absolute first beginning of it (s. A. IV, 77).

"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.LVI, 41).

Buddhist Dictionary, Manual of Buddhist Terms and Doctrines, by NYANATILOKA MAHATHERA

impenetrable : (adj.) avinivedhiya; anibbijjhiya; duravabodha.

A.P. Buddhadatta Mahathera, Concise Pali-English and English-Pali Dictionary [available as digital version from Metta Net, Sri Lanka]