Like 空, the kanji 虚 has the kun-yomi muna(shii), meaning “empty, vain, futile.” In fact, 虚 has several overlaps with 空:
• Another kun-yomi of 虚 is uro, meaning “cavity, hollow, hole.” Although 空 doesn’t mean “hole,” its top component, 穴, certainly does.
• The on-yomi of 虚 (KYO, KO) is similar to that of 空 (KŪ). Both on-yomi mean “empty.”
• The two characters even hook up in these compounds:
空虚 (kūko: emptiness, vacancy)
虚空 (kokū: empty space, sky)
Jim Breen’s online dictionary gives “untruth” as another definition of 虚. Indeed, as Kenneth Henshall notes in A Guide to Remembering Japanese Characters, 虚 (empty) combines with 口 (mouth) to form the character 嘘 (uso: lies). We’ve seen this kind of falseness in many 空 words.