firmament (n.) Look up firmament at Dictionary.com
mid-13c., from Latin firmamentum "firmament," literally "a support or strengthening," from firmus "firm" (see firm (adj.)), used in 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.