noun

• (geography) A small valley between mountains.
  "He built himself a cabin in a hollow high up in the Rockies."
• (figuratively) A feeling of emptiness.
  "a hollow in the pit of one’s stomach"

verb

• (transitive) to make a hole in something; to excavate

adj

• (of something solid) Having an empty space or cavity inside.
  "a hollow tree; a hollow sphere"
• (of a sound) Distant, eerie; echoing, reverberating, as if in a hollow space; dull, muffled; often low-pitched.
  "He let out a hollow moan."
• (figuratively) Without substance; having no real or significant worth; meaningless.
  "a hollow victory"
• (figuratively) Insincere, devoid of validity; specious.
  "a hollow promise"

adv

• (colloquial) Completely, as part of the phrase beat hollow or beat all hollow.