Meaning

noun the apparent surface of the imaginary sphere on which celestial bodies appear to be projected

originFrom Middle English firmament, furmament (“heaven; sky”), from Old French firmament (“firmament”), or from its etymon Latin firmāmentum (“support; sky”), from firmāre (“to strengthen”) + -mentum (suffix indicating an instrument or medium, or the result of an action). Firmāre is the present active infinitive of firmō (“to make firm, strengthen”), from firmus (“firm, strong, stable”), ultimately…

Etymology from Wiktionary, CC BY-SA 3.0