an upper room: a chamber built on top of a house or as the upper story of a larger building
The word names the room you reach by going up. In an ordinary Israelite house it was the roof chamber, a room built on the flat roof and reached by an outside stair. Cooler and more private than the rooms below, it served as a retreat, a guest room, or a quiet place to withdraw. This is the room where Elijah lodged and revived the widow's son, the room the wealthy woman of Shunem built and furnished for Elisha, and the chamber King Eglon sat in alone. It is also where David climbed to weep for Absalom, out of sight of everyone.
The same word stretches to the upper rooms and upper stories of grand buildings: the upper chambers of the temple overlaid with gold, the spacious upper rooms a king built for himself, a corner chamber set into the city wall. The core picture never changes, a space higher up that you ascend to, whether it tops a family home or a palace.
One last step lifts the word into the sky. Poetry pictures God building his own upper chambers above the waters of the heavens and watering the mountains from them. It is the ordinary architectural word, now describing the LORD's dwelling as a house whose upper rooms rest on the clouds.