Adam Clarke on Matthew 12:36

Every idle word - Ρημα αργον, a word that does nothing, that neither ministers grace nor instruction to them who hear it. The word αργον corresponds to the Hebrew שוא shave, which signifies not only vain or empty, but also wicked and injurious, such as a false testimony against a neighbor, compare Deu 5:11, Deu 5:20. Add to this, that Symmachus translates פגול piggul, polluted, Lev 19:7, by the very Greek word in the text. It was to explain this ambiguous meaning of the word, that ten MSS. have changed αργον into πονηρον, evil. Our Lord must be understood here as condemning all false and injurious words: the scope of the place necessarily requires this meaning.