My brethren - To wit, the three friends who had come to condole with him. He uses the language of brethren, to intimate what he had a right to expect from them. It is common in all languages to give the name brethren to friends.
Have dealt deceitfully - That is, I have been sadly disappointed. I looked for the language of condolence and compassion; for something to cheer my heart, and to uphold me in my trials - as weary and thirsty travelers look for water and are sadly disappointed when they come to the place where they expected to find it, and find the stream dried up. The simile used here is exquisitely beautiful, considered as a mere description of an actual occurrence in the deserts of Arabia. But its chief beauty consists in its exact adaptation to the case before him, and the point and pith of the reproof which it administers. "The fullness, strength, and noise of these temporary streams in winter, answer to the large professions made to Job in his prosperity by his friends. The dryness of the waters at the approach of summer, resembles the failure of their friendship in time of affliction." Scott, as quoted by Noyes.
As a brook - That is, as a stream that is swelled by winter torrents, and that is dry in summer. Such streams abound in Arabia, and in the East generally. The torrents pour down from the hills in time of rain, or when swelled by the melting of the ice; but in summer they are dry, or their waters are lost in the sand. Even large streams are thus absorbed. The river Barrady, which waters Damascus, after passing to a short distance to the southeast of the city toward the Arabian deserts, is lost in the sand, or evaporated by the heat of the sun. The idea here is, that travelers in a caravan would approach the place where water had been found before, but would find the fountain dried up, or the stream lost in the sand; and when they looked for refreshment, they found only disappointment. In Arabia there are not many rivers. In Yemen, indeed, there are a few streams that flow the year round, and on the East the Euphrates has been claimed as belonging to Arabia. But most of the streams are winter torrents that become dry in summer, or rivulets that are swelled by heavy rains.
An illustration of the verse before us occurs in Campbell's Travels in Africa. "In desert parts of Africa it has afforded much joy to fall in with a brook of water, especially when running in the direction of the journey, expecting it would prove a valuable companion. Perhaps before it accompanied us two miles it became invisible by sinking into the sand; but two miles farther along it would reappear and raise hopes of its continuance; but after running a few hundred yards, would sink finally into the sand, no more again to rise." A comparison of a man who deceives and disappoints one to such a Stream is common in Arabia, and has given rise, according to Schultens, to many proverbs. Thus, they say of a treacherous friend, "I put no trust in thy torrent;" and, "O torrent, thy flowing subsides." So the Scholiast on Moallakat says, "a pool or flood was called Gadyr, because travelers when they pass by it find it full of water, but when they return they find nothing there, and it seems to have treacherously betrayed them. So they say of a false man, that he is more deceitful than the appearance of water" - referring, perhaps, to the deceitful appearance of the mirage in the sands of the desert; see the notes at Isaiah 35:7.
And as the stream of brooks they pass away - As the valley stream - the stream that runs along in the valley, that is filled by the mountain torrent. They pass away on the return of summer, or when the rain ceases to fall, and the valley is again dry. So with the consolations of false friends. They cannot be depended on. All their professions are temporary and evanescent.

Have dealt deceitfully as a brook - There is probably an allusion here to those land torrents which make a sudden appearance, and as suddenly vanish; being produced by the rains that fall upon the mountains during the rainy season, and are soon absorbed by the thirsty sands over which they run. At first they seem to promise a permanent stream, and are noticed with delight by the people, who fill their tanks or reservoirs from their waters; but sometimes they are so large and rapid as to carry every thing before them: and then suddenly fail, so that there is no time to fill the tanks. The approach of Job's friends promised much of sympathy and compassion; his expectations were raised: but their conduct soon convinced him that they were physicians of no value; therefore he compares them to the deceitful torrents that soon pass away.

My brethren have dealt deceitfully as a (k) brook, [and] as the stream of brooks they pass away;
(k) He compares friends who do not comfort us in our misery to a brook which in summer when we need water is dry, in winter is hard frozen and in the time of rain when we have no need overflows with water.

My brethren have dealt deceitfully as a brook,.... Meaning his three friends, represented by Eliphaz, who were of the same sentiments with him, and behaved towards Job as he did: these were his brethren not by birth by blood nor by country, but by the profession of the same religion of the one true and living God in opposition to the idolatrous people among whom they dwelt; and this their relation to him is an aggravation of their perfidy and treachery, unfaithfulness and deceit, by which is meant their balking and disappointing him in his expectations; when they came to visit him as friends, he might reasonably expect they came to condole and sympathize with him, and comfort him; but, instead of this they reproached him and grieved him, and were miserable comforters of him; and this he illustrates by the simile of a "brook", which he enlarges upon in the following verses: these friends and brethren of his he compares to a "brook", not that was fed by a spring which continues, but filled with falls of water and melting snows from the hills, with which it is swelled, and looks like a large river for a while, but when these fail it is soon gone; hereby representing his friends in his state of prosperity, who looked big, and promised long and lasting friendship, but proved, in time of adversity, unfaithful and deceitful; and so it denotes the fickleness and inconstancy of their friendship:
and as the stream of brooks they pass away: or, "pass by" (g), as a stream of water, fed by many brooks, or flows of water like unto many brooks, which run with great rapidity and force, and are quickly gone and seen no more; thus his friends, as such, passed by him, and were of no use to him any more than the priest and Levite were to the man that fell among thieves, Luke 10:30.
(g) "praetereunt", Mercerus, Schmidt; "transeunt", Piscator, Cocceius, Michaelis.

Those whom I regarded as "my brethren," from whom I looked for faithfulness in my adversity, have disappointed me, as the streams failing from drought--wadies of Arabia, filled in the winter, but dry in the summer, which disappoint the caravans expecting to find water there. The fulness and noise of these temporary streams answer to the past large and loud professions of my friends; their dryness in summer, to the failure of the friendship when needed. The Arab proverb says of a treacherous friend, "I trust not in thy torrent" (Isaiah 58:11, Margin).
stream of brooks--rather, "the brook in the ravines which passes away." It has no perpetual spring of water to renew it (unlike "the fountain of living waters," Jeremiah 2:13; Isaiah 33:16, at the end); and thus it passes away as rapidly as it arose.

Brethren - Friends; for though Eliphaz only had spoken, the other two shewed their approbation of his discourse. Deceitfully - Adding to the afflictions which they said they came to remove. And it is no new thing, for even brethren to deal deceitfully. It is therefore our wisdom to cease from man. We cannot expect too little from the creature, or too much from the creator.