Origin: 1250–1300; Middle English < Late Latin firmāmentum sky, Latin: support, prop, stay, equivalent to firmā(re) to strengthen, support (see firm2) + -mentum-ment
mid-13c., from L. firmamentum "firmament," lit. "a support or strengthening," from firmus "firm" (see firm (adj.)), used in Vulgate to translate Gk. stereoma "firm or solid structure," which translated Heb. raqia, a word used of both the vault of the sky and the floor of the