Numbers 21:4

4.And they journeyed from mount Hor. This also is narrated in their praise, that they bore the weariness of a long and circuitous march, when they were already worn down by their wanderings for forty years. Moses, therefore, tells us that, since God had forbidden them to pass the borders of Edom, they went by another way; but immediately afterwards he adds, that they basely rebelled, without being provoked to do so by any new cause. They had before been rebellious under the pressure of hunger or thirst, or some other inconvenience; but now, when there were no grounds for doing so, they malignantly exasperate themselves against God. Some understand that they were afflicted in mind because of the way, (117) so that the ב, beth, indicates the cause of their grief and trouble. It might, indeed, be the case that their passage through the mountains was steep and difficult; but a pleasant region was almost in sight, gently to attract them onward. Again, they falsely complain of want of water, in which respect God had already applied a remedy. Nothing, then, could be more unfair than odiously to recall to memory a past evil, in which they had experienced the special aid of God. But their depravity is more thoroughly laid open in their loathing of the manna, as a food affording but little nutriment, or contemptible.

The verb (118) קצר, katzar, is used first, which signifies to constrain; thus some explain it, that they were rendered anxious by distress. But since the same word is used for to shorten, others translate it that their minds were broken down with weariness, so as to faint by the way. In any case, a voluntary bitterness is indicated, whereby they were possessed, so that their alacrity in advancing altogether failed them. The verb (119), קצה, katzah, which Jerome renders sickens, is not used simply for disgust, but signifies that weariness which excruciates or agonizes the mind.

They call the manna “light” food; as much as to say that it inflates rather than satisfies or nourishes; or, as I deem more probable, the word קלקל, kelokel, is used metaphorically for vile, or contemptible, and valueless.