Albert Barnes Commentary Job 12:21

"He poureth contempt upon princes, And looseth the belt of the strong." — Job 12:21 (ASV)

He pours contempt upon princes - He has power to hurl them from their thrones, and to overwhelm them with disgrace.

And weakens the strength of the mighty - Margin, as in Hebrew, loosens the girdle of the strong. The Orientals wore loose, flowing robes, which were secured by a girdle around the loins. When they labored, ran, or traveled, their robes were girded up.

But this practice is common everywhere. Wrestlers, leapers, and runners put a girdle around them and are thus able to accomplish much more than they otherwise could. To loosen that girdle is to weaken them.

So Job says that God had power to loosen the strength of the mighty. He here seems to labor for expressions, and varies the form of the image in every way to show the absolute control God has over people, and the fact that His power is seen in the reverses of humankind. Lucretius has a passage strongly resembling this in its general sentiment:

Usque adeo res humanas vis abdita quaedam
Obterit; et pulchros fasces, saevasque secures,
Proculcare, atque ludibrio sibi habere, videtur.
Lib. v. 1232.

So from his awful shades, some Power unseen
O’erthrows all human greatness! Treads to dust
Rods, ensigns, crowns - the proudest pomps of state;
And laughs at all the mockery of mad!
Good.