Source: p 216, Philosophy: The Classics (4 ed, 2014) by Nigel Warburton PhD in Philosophy (Cambridge)
Kant distinguishes between the world we experience (the world of phenomena), and the underlying reality behind it. The underlying reality consists of noumena, about which we can say nothing at all because we have no access to them. We are restricted to knowledge of phenomena; noumena must for ever remain mysterious to us. Hence most metaphysical speculation about the ultimate nature of reality is misguided, since it purports to describe features of the noumenal world, and our lot is to dwell entirely in the phenomenal one.
[Wikipedia :] The Greek word νοούμενoν nooúmenon, plural νοούμενα nooúmena, is the neuter middle-passive present participle of νοεῖν noeîn "to think, to mean", which in turn originates from the word νοῦς noûs, an Attic contracted form of νόος nóos "perception, understanding, mind".
Kant's use of the Loan Word 'noumenon' appears to contradict its original meaning in Greek; to what thinking or perception can nooúmenon refer, if humans can never perceive, understand, or think about noumena? Did Kant intend a semantic shift?