clinker

(klĭngˈkər)

1. The incombustible residue, fused into an irregular lump, that remains after the combustion of coal.
2. A partially vitrified brick or a mass of bricks fused together.
3. An extremely hard burned brick.
4. Vitrified matter expelled by a volcano.
5. Slang
   a. A sour note in a musical performance: hit a clinker.
   b. A mistake; a blunder.
   c. Something of inferior quality; a conspicuous failure: a clinker of a show.
6. Chiefly British Something admirable or first-rate.