Green’s Dictionary of Slang

(UK teen) one’s best friend.

2016		Observer (London) Rev. 23 Oct. 28/2: Paxman’s [...] interview with Trump’s new bestie, Nigel Farage.
2016		T. Robinson Rough Trade [ebook] Twitch walked back downstairs to break the news to his new besties.
2017		T. Spicer Good Girl Stripped Bare 216: Lisa, my bestie from uni.
2022		J. Hannaham Didn’t Nobody Give a Shit 152: He ain’t make it sound like they was besties or nothing.