accomplice; conspirator; cohort
Colloquial term for a partner in crime or someone in on a scheme together. Often used in the phrase ぐるになる (to be in cahoots).
Those two were in cahoots and tricked me.
Turns out the manager was in on it too.
It turned out he was an accomplice in the robbery.
Hiragana spelling is also used, especially in informal writing.
The origin is uncertain. It may be related to the verb 繰る (くる, to reel or wind), suggesting people bound together in a plot, but this is speculative.