The Greek adjective asunthetos means covenant-breaker — one who violates agreements and compacts. Built on sunthetos (a compact, that which is put together) with the alpha-privative: one who undoes what was bound together, who dissolves covenant bonds.
Breaking covenant — with God and with one another — is the fundamental human sin in biblical theology. The Mosaic covenant's curses fall on those who break it (Deuteronomy 29). The prophets rail against Israel's covenant unfaithfulness. But in Christ, God keeps the covenant humanity breaks: He is the faithful covenant partner who fulfills every promise. The New Covenant in His blood establishes a relationship that cannot be dissolved by human faithlessness — because it rests entirely on the faithfulness of God.