also: くうげん · むなこと
空言を言うな。
Don't tell lies.
虚言 read きょげん is a near-synonym, also meaning 'falsehood', but is more formal and often used in written language.
Standard kanji spelling for the common reading そらごと.
Rare reading restricted to the 空言 spelling; mainly in Buddhist or classical contexts.
Alternate kanji spelling; also read きょげん with a similar meaning.
Rare kana-only spelling for the reading むなこと; archaic or dialectal.
Compound of 空 (sora, 'empty, sky') + 言 (koto, 'word, speech'), with rendaku voicing to goto. The literal meaning is 'empty words', i.e., words without substance.