clinker, n.⁶ — Green’s Dictionary of Slang

clinker n.⁶

[SE clinker, a very hard brick; ? since its ult. ety. is Du., as imported by Dutch immigrants]

(US) a hard biscuit.

1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 28: clinker, n. A biscuit.
1912		Ade Knocking the Neighbors 63: Members of the Middle and Lower Classes of the Animal Kingdom, who ate Sponge Cake with Clinkers in it.
1920		Ade Hand-made Fables 40: Every Tuesday they served Dried Currants with Clinkers in them.
1936		H.W. Bentley ‘Linguistic Concoctions of the Soda Jerker’ in AS XI:1 42: CLINKER. Biscuit.