To call something unthinkable is to say that it is too unpleasant or too abstruse to think about or that no amount of thinking about it will yield a clear understanding of it. The Buddha uses the term unthinkable (acinteyya) in this last sense.
He said, ‘There are these four unthinkables, not to be thought about, thinking about which one will become agitated and confused. What four? Thinking about the range of a Buddha’s understanding, thinking about the range of some meditative states, thinking about the results of kamma and thinking about the origins of the world’ (A.II,79).
Paradoxically, a great deal of Buddhist commentarial writing has been devoted to the first three of these subjects. Buddhist theoreticians and abhidhamma thinkers of the past speculated endlessly and argued with one another about the nature of the Buddha’s realization and about what past action would have caused what kammic results.