Green’s Dictionary of Slang

prong n.

also pronger

1. [mid-19C; 1940s+] the penis, esp. when erect.

2. [mid-19C] a finger.

3. [1930s] (drugs) a hypodermic syringe.

In compounds

prong-on (n.)

[1960s+] an erection.

In phrases

get the prong (v.)

[1910s] (US) to suffer.