To hold a thing well enough to act rightly on it. Understanding is not information; it is fit between a mind and a thing.
Old English understandan, from under (among, before) and standan (to stand), literally to stand in the midst of a thing, named holding a matter well enough to act on it.
Holding a thing well enough to act rightly upon it, a fit between a mind and a matter rather than a stock of information about it.