Dan here at UltiMAK told me recently that the word “understand” comes from building, or dwelling construction, long ago. If you were going to add a floor atop an existing structure, you would need to “understand” the structure so as to prevent catastrophic failure (that is to say; support with additional stands underneath). If your building collapsed, you failed to “understand”.
That would make our current usage another of the many euphemisms that we no longer know to be euphemisms (at least I never knew – maybe you all heard it before, or maybe it’s not true). It would also add a layer of perception to the word. If you fail to “understand” you failed to acquire, or to take the time to construct, the requisite foundation and lower level structure to support what you just saw or heard, and so it collapses in on you.