Green’s Dictionary of Slang

conk n.2

[Congolene, a fiery liquid, combining lye, eggs, potatoes and other ingredients, used in the artificial straightening of naturally kinky black (racially) hair]
[1930s+] (US black)

1. hair that has been straightened through the application of a special mixture.

2. pomade; hair grease.

In compounds

conk job (n.)

[1960s+] (US black) a black hairstyle that has been straightened.

conkhead (n.)

[1950s] (US black) a black person.