Green’s Dictionary of Slang

side v.2

[side n.]

to act in a pretentious manner, to put on airs.

[UK]Wodehouse Mike [ebook] ‘[T]here’s just a chance you might start to side about a bit soon, if you don’t watch yourself’.
[UK]A. Lunn Harrovians 47: He waits till he finds him wearing carpet slippers in the yard, and then tells him he’s siding.

In phrases

sidebust (v.)

(US black) to gossip, to tell tales.

[US]L. Bing Do or Die (1992) 120: Okay — this is Gang Class. There’s no sidebusting in here. Anything said in this room stays here.