To be in cahoots with someone is to share a partnership or to work together toward a common goal. The word’s exact derivation is unknown, though it’s apparently an Americanism from the early 19th century, and it may relate to the French cahute, meaning cabin or hut. The word was originally used singularly (e.g., he is in cahoot with me), but it’s now almost always plural.
Cahoots often has sinister connotations, suggesting conspiracy, corruption, or criminality on the part of those in cahoots with each other.
Examples
Of course, politicians and crooks being in cahoots is not unique to India. [Wall Street Journal]
Every passing week sees land confiscated from villages by a corrupt developer in cahoots with a local party official. [NPR]
The students found to be in cahoots were sent to in-school suspension and forced to call their parents. [Victoria Advocate]
Even so, the students claimed that the hospital was in cahoots with the police in covering up the injury. [Malaysian Insider]