shaḥaq

This is a poetic word for the sky, almost always used in the plural and almost only in poetry (Psalms, Job, and the prophets). It never turns up in plain prose, where a writer reaches instead for the everyday words for cloud (עָנָן) or heaven (שָׁמַיִם). The picture behind it comes from a root that means to grind or beat something fine: the sky is imagined as a thin sheet hammered out across the heights, exactly the image in Job, where God spreads out the skies hard as a cast-metal mirror.

In use it slides between two closely linked ideas. Sometimes it names the clouds themselves, the masses overhead that hold water and release it: they drip on people, pour down showers, and rumble with thunder. Sometimes it names the sheer height of the sky, the lofty vault where God rides and reigns. This is why the poets say that God's faithfulness and truth reach up to the skies: the word marks the highest reachable place, a way of picturing something without limit.

Close neighbors carry different weight. עָב is the thick, dark storm cloud heavy with rain, and the poets pair the two, singing of the dark clouds (עָב) of the skies. עָנָן is the ordinary cloud, including the cloud of God's presence in the desert. This word is the loftier, sweeping term, reaching for the whole cloud-filled height of heaven at once.

In one place the word steps back to its root meaning. The root behind it means to grind or pulverize, and here it names the fine powder that grinding leaves behind. Isaiah pictures the nations weighed on a merchant's scale and found to be nothing: not even a full weight, but the barely-there film of dust clinging to the pan. It is the same word used elsewhere for the thin-beaten sky, now pointed back at its plain sense of something pounded fine.