Albert Barnes Commentary Matthew 12:36

"And I say unto you, that every idle word that men shall speak, they shall give account thereof in the day of judgment." — Matthew 12:36 (ASV)

But I say unto you, etc. Christ closes this address to his malignant and wicked hearers by a solemn declaration that for these things God would bring them into judgment. They, therefore, who had spoken so malignantly against him, could not escape.

Idle word. This literally means a vain, thoughtless, useless word, that accomplishes no good. Here it means, evidently, wicked and injurious, false and malicious; for such were the words which they had spoken.