RhymeZone: underly definitions

Definitions of underly:

adjective: (dialectal) Poor; inferior.
adverb: (rare, obsolete) At a lower level.
adverb: (chiefly colloquial) In an inferior or inadequate manner; insufficiently (chiefly in opposition to overly).
verb: Nonstandard spelling of underlie. [(intransitive) To lie in a position directly beneath something.]

(Definitions from Wiktionary)