marked by firm determination or resolution; not shakable; "firm convictions"; "a firm mouth"; "steadfast resolve"; "a man of unbendable perseverence"; "unwavering loyalty"
If someone is steadfast in something that they are doing, they are convinced that what they are doing is right and they refuse to change it or to give up. He remained steadfast in his belief that he had done the right thing
firm and dependable especially in loyalty; "a steadfast ally"; "a staunch defender of free speech"; "unswerving devotion"; "unswerving allegiance"
[ 'sted-"fast also -f& ] (adjective.) before 12th century. From Middle English stedefast which in turn comes from Old English stedefæst. It is comprised of two parts: stede (“stead, place”) + -fæst (“fixed”).