firmament (n.)

mid-13c., "expanse of space above the earth where the heavenly bodies move," hence "the arch or vault of the heavens, the visible sky" (c. 1300), from Old French firmament or directly from Latin firmamentum "firmament," literally "a support, a strengthening," from firmus "strong, steadfast, enduring" (from suffixed form of PIE root *dher- "to hold firmly, support" ).

Used in Late Latin in the Vulgate to translate Greek stereoma "firm or solid structure," which translated Hebrew raqia, a word used of both the vault of the sky and the floor of the earth in the Old Testament, probably literally "expanse," from raqa "to spread out," but in Syriac meaning "to make firm or solid," hence the erroneous translation.

In Ptolemaic astronomy, "the sphere of the fixed stars" (c. 1300). Related: Firmamental.