Define steadfast. steadfast synonyms, steadfast pronunciation, steadfast translation, English dictionary definition of steadfast. also sted·fast adj. 1. Firmly loyal or constant; unswerving. The adjective 'steadfast' has its etymological origins in Old English. It is derived from the Old English word 'stedefæst,' which combines 'stede' meaning 'place' or 'stead' and 'fæst' meaning 'fast' or 'firm.' In its original form, 'steadfast' described something that was firmly fixed in a particular place or position. steadfast meanings, definitions, synonyms, and antonym. Explore more Positive Words at Dictionary by BUHAVE. The meaning of STEADFAST is firm in allegiance to a person, belief, or cause : loyal, unswerving. How to use steadfast in a sentence. Did you know? Synonym Discussion of Steadfast.