clinker Definition

clinker¹
noun

1. A mass of fused ash or slag left unburnt in a furnace.
2. The cindery crust on a lava flow.
3. A very hard kind of brick.

Etymology: 17c: from Dutch klinker hard brick, from klinken to ring.

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clinker²
noun

slang:
(Brit)
1. Something or someone that is popular or outstanding.
(chiefly US)
2a. A mistake or failure;
(chiefly US)
2b. Something of poor quality.