also: からしょうもん
noun
empty promise; false pledge
Refers to a promise or written pledge that is not meant to be kept; often used in contexts of deception or insincerity.
彼の言ったことは結局空証文だった。
What he said turned out to be an empty promise.
そんな空証文を信じてはいけない。
You mustn't believe such a false pledge.
空手形 is a more common term for an empty promise or a bad check, while 空証文 is rarer and more literary.
Compound of 空 (そら, 'empty, false') and 証文 (しょうもん, 'written pledge, deed'). The exact historical derivation is uncertain, but the word literally means 'empty written pledge'.