The Collaborative International Dictionary of English v.0.48:

Cahoot \Ca*hoot"\, n. [Perhaps fr. f. cohorte a company or
   band.]
   Partnership; league; as, to go in cahoot (or in cahoots) with
   a person. Usually used in the plural, and in modern usage
   often used to imply that the joint effort is unethical,
   shady, questionable, or illegal; as, a shill in cahoots with
   a pickpocket, to serve as a distraction. [Slang, southwestern
   U. S.] --Bartlett.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

cahoot
    n 1: collusion; "in cahoots with"