Albert Barnes on Matthew 12:36

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.

(h) "give account" Eccl 12:14, Eph 5:4,6, Jude 1:15