Abarim Publications' online Biblical Hebrew Dictionary
The root-verb רסן (rsn) isn't used in the Bible, so we don't know what it may have meant. The only indication that it probably existed in the Hebrew language (but simply was never incorporated in the text of the Bible) is the existence of the masculine noun רסן (resen), possibly meaning jaw primarily and secondarily, but most often, used in the sense of halter. It occurs four times in the Bible; Job 41:5 either speaks of Leviathan's jaws (BDB Theological Dictionary) or of Leviathan being bound (New American Standard). Psalm 32:9, Job 30:11 and Isaiah 30:28 clearly speak of some kind of restraint, like a bridle or halter.