Does it count as an oath if I say to someone, while narrating a story about a quarrel: "By divorce, you are an idiot," knowing that I did not actually say that in reality?

Your false statement that your wife's divorce is suspended does not cause it to take effect. You must repent to Allah for lying, as it is a grave sin and a reprehensible trait. Its prohibition has been established, and warnings have been issued concerning it, because it leads to sins and evil deeds, and distances one from uprightness in obedience. The Prophet, peace and blessings be upon him, said: "Indeed, truthfulness leads to righteousness, and righteousness leads to Paradise. And a man keeps on telling the truth until he becomes a Siddiq (truthful person). And indeed, lying leads to wickedness, and wickedness leads to the Hellfire. And a man keeps on lying until he is written with Allah as a liar."