na-adjective, noun
Describes a state of emptiness or hollowness, often in a philosophical or abstract sense. Can be used as a noun or adjectival noun.
彼の心は空空としていた。
His mind was empty.
空空とした部屋には何もなかった。
There was nothing in the vacant room.
空虚 also means emptiness or void, but is more commonly used in modern Japanese and can carry a nuance of futility.
Reduplication of 空 (くう, 'emptiness, void'), emphasizing the state of being empty. The exact historical derivation is uncertain.