in league with

agreeing to do something with someone else The accountant and the chairman were in league to hide the company's debts. I believe my children are in league with the devil!

Also, in cahoots with. In close cooperation or in partnership with, often secretly or in a conspiracy. For example, "For anybody on the road might be a robber, or in league with robbers" (Charles Dickens, A Tale of Two Cities, 1859), or We suspect that the mayor is in cahoots with the construction industry. The first term dates from the mid-1500s. The variant, a colloquialism dating from the early 1800s, may come from the French cahute, "a small hut or cabin," and may allude to the close quarters in such a dwelling.