Part of speech: noun

Definition:  A police officer’s truncheon.

Source: Oxford Dictionary

Found in “Field Trip to the Museum of Human History” by Franny Choi, line 9, fifth stanza

a “nightstick,” so called for its use/ in extinguishing the lights in one’s eyes.”

I don’t have much experience with law enforcement so didn’t know what a night stick was. I first assumed the word nightstick meant a flashlight or some sort of device used to blind people by the context of the poem. Knowing it’s actually another word for the club policemen carries give the following line a much darker context since its not speaking of policemen blinding people literally, but explaining how they would “knock someone’s light out”, beat them till they were unconscious.