Green’s Dictionary of Slang

1. (drugs) to reach the most extreme point of an LSD trip.

1971		Current Sl. V:4.
1997		(con. 1970s) G. Pelecanos King Suckerman (1998) 152: He felt like he was peaking in a red-and-gold tinted trip.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 137/2: peak (also peak out) v. to hit the height of one’s drug-induced ‘high’.
2002		T. Dorsey Triggerfish Twist (2002) 87: ‘How do you feel?’ asked Bernie. Coleman looked slowly around the room. ‘[...] floating, flying, peaking, sailing, [...].’.
2004		L. Redhead Peepshow [ebook] I’d danced for a bucks’ party off their brains on Es, everyone peaking so badly their eyes were rolling back.

2. to reach the limit of a particular experience.

1982		J. Ellroy Clandestine 20: We kissed hard and long, then Sarah opened up [...] and arched to receive me. [...] We peaked together.
1984		in G. Hirshy Nowhere to Run 157: I feel I’m peaking [...] Yes, that’s it. You can say I’m in my peak experience.
1999		L. McLean Riddance 29: kenny: You’re looking good, Frank. frank: I know. I’m peaking. kenny: Peaking? frank: Getting as good as I’m going to get.
2007		K. Moore Bowerman 284: I’m peaking perfectly. God, I wish we were racing the Olympics tomorrow!