adjective

1. having an empty space, or only air, within it; having a cavity inside; not solid
2. depressed below the surrounding surface; shaped like a cup or bowl; concave
3. deeply set; sunken hollow cheeks
4. empty or worthless; not real or meaningful hollow praise
5. hungry
6. deep-toned, dull, and muffled, as though resounding from something hollow

Etymology: ME holwe < OE holh: see hole

adverb

noun

1. a hollow formation or place; cavity; hole
2. a small, sheltered valley

transitive verb, intransitive verb