Matthew 12:36
I tell you that every idle word that men speak, they will give account of it in the day of judgment.
WEB
But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
KJV
What Matthew 12:36 means
Jesus says even our careless, throwaway words reveal what is really inside us, so they carry weight and will count in the final reckoning.
Jesus says this in the middle of an argument with religious leaders who had just accused him of doing his work by the power of evil. He has been making the point that a tree is known by its fruit, and that people say what their hearts are full of. An "idle" word here means a careless or throwaway one, the kind we assume does not count.
His point is that words are evidence. They show what is actually inside a person, so even the offhand ones carry weight and belong in the final reckoning. The next line says it plainly: by your words you are justified, and by your words you are condemned.