Theophylact of Ohrid

So also the tongue, a world of iniquity, is fire, for as fire destroys everything, so does the tongue. Some understand "world" here as multitude, as also in the words: "the world did not know Him" (John 1:10). The tongue is a world, that is, a mass of iniquity. Thus the tongue, setting ablaze the wheel of birth, defiles life. Life is also called a crown by the psalmist, when he says to God: "You shall bless the crown of the year of Your goodness" ("You crown the year with Your goodness") (Ps. 65:11). …

Commentary on James ↗

Oecumenius

And the tongue is fire, a whole world of iniquity. The tongue is set among our members, which defiles the whole body, and inflames the wheel of our birth, inflamed of hell. "world of iniquity." In this place, some wish to signify the multitude, just as where it is said: "The world did not know him," (Jn. 1:10) that is, the popular multitude. Therefore, the world is also the tongue, that is, the multitude of iniquity. And how great if a multitude was said there when it was sent before, how great the matter.

Commentary on James ↗

Bede

And it sets on fire the wheel of our birth, being set on fire by hell. By hell, it means by the devil and his angels, for whom hell was made, and who, whether they fly in the air, wander on earth or under the earth, or are detained, always bring with them the torments of flames, like a feverish person who, even if placed on ivory beds or in sunny places, cannot avoid the heat or cold of his inherent disease. …

Commentary on the Catholic Epistles ↗