Underlying means lying beneath the surface and explaining it. The underlying cause, underlying conditions, underlying assumptions.
It claims that what you can see is not the whole story, which is why it appears in every explanation of a problem: underlying causes, underlying trends. In medicine, underlying conditions are the ones that make another illness worse. In finance, the underlying is the asset a derivative is based on. Note underlie, whose past is underlay.
1adjectiveunderlyingLying beneath the surface and explaining it.
“The doctor wanted to find the underlying cause of her headaches.”
underlying is a quality of being beneath.
Old English
underliċġan — to underlie, to be subject to, give way to
Middle English
underlien
English
underlie
English, today
underlying
by surface analysis, underlie + -ing. underlie comes from Middle English underlien, underliggen, from Old English underliċġan (“to underlie, to be subject to, give way to”), equivalent to under + lie. Cognate with Dutch onderliggen (“to lie below, lie on the bottom of”), German unterliegen (“to lie under, be subject to, succumb”).
Etymology from Wiktionary, CC BY-SA