Clinker \Clink"er\ (kl[i^][ng]k"[~e]r), n. [From clink; cf. D. clinker a brick which is so hard that it makes a sonorous sound, from clinken to clink. Cf. Clinkstone.]

1. A mass composed of several bricks run together by the action of the fire in the kiln.

2. Scoria or vitrified incombustible matter, formed in a grate or furnace where anthracite coal in used; vitrified or burnt matter ejected from a volcano; slag.

3. A scale of oxide of iron, formed in forging.

4. A kind of brick. See Dutch clinker, under Dutch.

Source: Webster's Revised Unabridged Dictionary (1913)