It's probably best to look at the structure of the word first. onverstandig comes from on- (“un-”) + verstandig (“sensible, wise”). The latter in turn derives from verstand (“reason, mind, understanding”), which finally is closely tied to verstaan (“to understand”). So you would need to follow this structure in English too.
However, the first hurdle is already that English uses a slightly different root word, understand. The second is that English does not have a noun paired with this word in the same way that Dutch has, unless you use understanding. But this doesn't allow an adjective to be derived from it in the same way, something meaning "of or related to understanding"... understandingy just doesn't really cut it.
Another approach is to look for synonyms of any of the intermediate steps. Starting from the end, you might translate verstand with mind, and following the process then gives mind(l)y and unmind(l)y. But you can also translate verstandig directly, giving wise, and then of course you simply end up with unwise which is a perfectly good translation of the Dutch onverstandig. :)