key, v. — Green’s Dictionary of Slang

key v.

(orig. US campus) to scratch an automobile with a key or other pointed object.

1989		P. Munro Sl. U.
1993		Tarantino & Avery Pulp Fiction [film script] 33: You know what some fucker did to it [i.e. a car] the other day? [...] Fuckin’ keyed it.
2005		P. Howard Curious Incident of the Dog in the Nightdress 36: If that jammer [...] hasn’t been keyed by the time I come back, there’s ten sheets in it for you.
2012		(con. 1980s) I. Welsh Skagboys 335: Ah start tae idly key a smart estate motor.
2012		R. Milward Kimberly’s Capital Punishment (2023) 12: I’ve often wondered why people like to [...] badmouth you, or key your car.
2016		T. Robinson Rough Trade [ebook] ‘He keyed Dana’s car, he smashed our window’.

In derivatives

keyed up (adj.)

(Aus. drugs) under the influence of cocaine, thus aggressive.

1932

Sun. Mail (Brisbane) 13 Nov. 20/7: ‘Joe was buckled last night. He was all keyed up with angie and tried to take a twist out of a demon, he dug his heels in and it took three of them to lumber him.’ [...] Joe was under the influence of cocaine. He used insulting language to a detective, and resisted so violently when placed under arrest that it took the detective and two other officers to remove him to the watch house.

In phrases

key into (v.)

(US) to initiate a newcomer into a situation.

1978

(con. 1964) J. Carroll The Basketball Diaries 39: [A] lot of the more lame dudes in the class didn’t really know what a homo was [...] (though I certainly had been keyed into that scene already by our coach at the Boys’ Club, Lefty, and various other dick snatchers.