An idiom meaning to do the opposite or contradict immediately after saying something; hardly had the words been said.
先週は来ないって言ったのに、舌の根も乾かぬうちに来ると言い出した。
He said last week he wouldn't come, but hardly had the words been said before he announced he'd come.
社長は値上げはしないと言ったが、舌の根も乾かぬうちに価格改定を発表した。
The president said there would be no price increases, but hardly had he said that before announcing a price revision.
彼女はダイエットを始めると言ったが、舌の根も乾かぬうちにケーキを食べていた。
She said she'd start a diet, but hardly had she said it before she was eating cake.
政治家の約束が舌の根も乾かぬうちに覆されるのはよくあることだ。
It's common for politicians' promises to be overturned hardly after they have been made.