Probably from cohort, Spanish and French, defined in the old French and English Dictionary of Hollyband, 1593, as "a company, a band." It is used at the South and West to denote a company or union of men for a predatory excursion, and sometimes for a partnership in business.
If I could only get the township and range, I'd make a cahoot business with the old man.--Simon Suggs, p. 37.
Pete Hopkins aint no better than he should be, and I wouldn't swar he wasn't in cahoot with the devil.--Chronicles of Pineville, p. 74.
I'd have no objection to go in cahoot with a decent fellow for a character, but have no funds to purchase on my own account.--New Orleans Picayune, p. 136.
The hoosier took him aside, told him there was a smart chance of a pile on one of the (card) tables, and that if he liked, he would go with him--in cahoot!--Field, Western Tales, p. 198