איוב מ:כג פירוש: רש״י ומלבי״ם

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.

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.

and [he] does not harden [His belly does not harden from his drinking, like (verse 17), “His tail hardens.”