firmament

From 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 from Proto-Indo-European *dʰer- (“to hold; to support”).

出典: Wiktionary「firmament」（CC BY-SA 4.0）