The Greek adjective hedraios (ἑδραῖος) means steadfast, firm, stable, or settled — like something seated solidly on a foundation. It derives from hedra (seat/base) and appears three times in the New Testament. It describes the quality of being unshakeable — not moved from one's position by pressure or persuasion.
Hedraios appears in two powerful calls to steadfastness. In 1 Corinthians 7:37, a man who is 'settled in his heart' (hedraios) makes wise decisions about marriage. More significantly, in 1 Corinthians 15:58 — immediately after Paul's great resurrection chapter — he charges believers to be 'steadfast, immovable, always abounding in the work of the Lord.' The resurrection is the foundation (hedra) of Christian stability; because Christ is risen, the work of the Lord has eternal weight. In Colossians 1:23, hedraios describes continuing in faith, 'firm and steadfast' — the stable posture of someone who has found their sure footing in Christ.