Definition of underlaying:
(v) :
(transitive) To lay (something) underneath something else; to put under.
(v) :
(transitive) To provide a support for something; to raise or support by something laid under.
(n) :
A soft floor covering that lies under a carpet.
(n) :
A layer (of earth, etc.) that lies under another; substratum.
(n) :
Anything that is underlaid.
(v) :
(transitive) To put a tap on (a shoe).