Origin: Germanic Old English prefix
Underneath has 6 different meanings across 2 categories:
The lower surface or part of something.
"The label was stuck on the flat top, but underneath it remained smooth and blank."
In plain English: Underneath is not really used as a noun, but when people say it that way they are usually referring to something hidden or located below another thing.
"The secret was hidden underneath the false bottom of the suitcase."
Under, lower.
"The cat curled up underneath the table to stay out of the rain."
on the lower or downward side; on the underside of
"a chest of drawers all scratched underneath"
under or below an object or a surface; at a lower place or level; directly beneath
"we could see the original painting underneath"
"a house with a good foundation underneath"
Below; in a place beneath.
"The cat curled up underneath the old sofa to stay warm."
In plain English: Underneath means being on the bottom side of something, often hidden from view by what is placed above it.
"The sun warms my back while I lie underneath the tree."
Under, below, beneath.
"The cat curled up underneath the table to sleep."
The word underneath comes from the Middle English term undernethe, which was formed by combining "under" with "neath." This construction has been used in English since at least the 14th century to mean the position below something else.