From meta (implying change of direction) and agō (to lead). To lead in a different direction, to turn, to guide. James uses this word of the bit in a horse's mouth and the rudder of a ship.
James illustrates the tongue's disproportionate power by comparing it to a horse's bit and a ship's rudder — small instruments that direct much larger bodies (James 3:3-4). A tiny rudder turns a great ship wherever the pilot desires. Similarly, the tongue — small as it is — directs the course of a human life. Words have directional power; they steer lives toward life or death.