Having a cavity, gap, or space within: a hollow wall. Deeply indented or concave; sunken: "His bearded face already has a set, hollow look\u201d ( Conor Cruise O'Brien). Without substance or character: a hollow person. See Synonyms at vain. Devoid of truth or validity; specious: "Theirs is at best a hollow form of flattery\u201d ( Annalyn Swan). Having a reverberating, sepulchral sound: hollow footsteps. A cavity, gap, or space: a hollow behind a wall. An indented or concave surface or area. A void; an emptiness: a hollow in one's life. A small valley between mountains. To make hollow: hollow out a pumpkin. To scoop or form by making concave: hollow out a nest in the sand. To become hollow or empty.