An idiom meaning that someone does or says something immediately after having just said the opposite; 'hardly had the words been said.'
彼は「明日は行けない」と言ったが、舌の根の乾かぬうちに予定を変えた。
He said, "I can't go tomorrow," but hardly had he said that before he changed his plans.
先生が注意した舌の根の乾かぬうちに、また同じミスをした。
Hardly had the teacher warned (them) than they made the same mistake again.
「行かない」と言っておきながら、舌の根の乾かぬうちに来た。
He said, "I won't go," yet came back almost immediately afterward.