A piece of slag or waste matter left after coal burns; also informally means something excellent, or in cricket/sports, a mistake or mishit.
From 'clink' (Middle Dutch klinken) plus agent suffix '-er', originally referring to the clinky residue from fires; later slang evolved with multiple meanings.
Clinker in cement production is actually what you want—it's the useful intermediate product made by heating limestone and clay that gets ground into cement powder, so industrial 'clinker' is valuable waste!
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