Cahoots
Cahoots is a noun that refers to colluding or conspiring together secretly. The origin of cahoots is unknown, but we know it emerged in the early 19th century and it’s pretty always used in the plural. Example: I quit my job when I discovered that my boss was in cahoots with the company executives. But I might have reacted differently if I’d known that they were secretly conspiring a surprise birthday party for me.