Pronunciation: (un"dur-lī'),[key] —v.t.-lay, -lain, -ly•ing. 1. to lie under or beneath; be situated under.
2. to be at the basis of; form the foundation of.
3. Gram.to function as the root morpheme or original or basic form of (a derived form): The form “boy” underlies “boyish.” 4. Finance.to be primary to another right or security.