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

unthinkable : (adj.) acintiya.

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

acintya[acintiya] unthinkable. Acintya is that which is beyond the thinking faculty of human beings, the thing they should not brood over due to the limits of the human thought, and also because such thinking does not lead to enlightenment. The four unthinkables are

(1) the sphere of a Buddha (buddha-viṣaya) [buddha visaya],
(2) meditative absorptions (dhyāna viṣaya) [jhāna visaya],
(3) karma result (karma-vipāka) [kamma vipāka],
(4) brooding over the world such as its beginning, whether it is eternal or finite, etc.(lokacintā), [lokacintā].

Buddhānusmṛti - A Glossary of Buddhist Terms