just beneath; directly under; one step below
Used for a place, area, or surface that is immediately below something else, or for being one rank/step lower in a hierarchy or comparison.
There is a basement just beneath this floor.
His ability is one step below professional level.
Compound of 一枚 (one sheet/layer) and 下 (below). The exact historical derivation is uncertain, but the meaning is transparent from its components.