הֵ֤ן יַעֲשֹׁ֣ק נָ֭הָר לֹ֣א יַחְפּ֑וֹז יִבְטַ֓ח ׀ כִּֽי־יָגִ֖יחַ יַרְדֵּ֣ן אֶל־פִּֽיהוּ׃
Behold, if a river overflow, he trembleth not; He is confident, though the Jordan rush forth to his mouth.
Rashi on Job
Behold, he plunders the river, and [he] does not harden The Euphrates, which is a large river whose waters increase, he oppresses and plunders from the creatures by his drinking.
Malbim on Job
The second mighty beast that Job should put down is the Leviathan. Beyond discipline or training, the Leviathan is the embodiment of ferocious and ungovernable violence; and it knows it and does not care. Many desperate attempts have been made to catch this rampaging sea-monster but all have been in vain. And even were it caught alive it would never submit; and if dead, its carcass would be of no use, as its flesh is too tough to be butchered.
Rashi on Job
and [he] does not harden [His belly does not harden from his drinking, like (verse 17), “His tail hardens.”