I don't remember where or when I heard this, but: Reprobating, Restraining, and Regenerated... an awkward attempt to express the law's function as condemnation to the unsaved, civil restraint to the world at large, and guidelines / path markers for the converted.
Mine is not one-word succinct, but I think it's understandable and memorable for a layman, plus it's Christ-focused:
1. It shows us how much we need Jesus.
2. It teaches us how to be like Jesus.
3. It keeps the world livable until Jesus returns.
This also helps me connect the law to key elements of salvation (justification, sanctification, glorification), avoiding a too-severe law/gospel distinction.