From an unused root meaning to curb; a halter (as restraining); by implication, the jaw -- bridle.
H7448. resen
I. רֶ֫סֶן noun masculineIsaiah 30:28:
1 halter;
2 jaw (?); —
1 absolute ׳ר Psalm 32:9 restraining mouth of horse or ass (+מֶתֶג); figurative of עַמִּים׳ר עַללְֿחָיֵי י, ׳שֵׁם Isaiah 30:28; ׳ר שִׁלֵּ֑חוּ מִמָּנַי Job 30:11, i.e. threw off restraint.
2 suffix רִסְנוֺ כֶּפֶל Job 41:5 the double of his jaw, his double jaws (of croc.), si vera 1.; connection of meanings strange, "" לְבוּשׁוֺ מְּנֵי, whence GHBW right Du conjecture סִיֹנוֺ.