wallow in the mire together;act in collusion;work hand in glove with;birds of a feather flock together.
沆瀣一气hànɡ xiè yī qì
沆瀣:原意为夜间的露水和水气,此指两个人的名字,一为“沆”,一为“瀣”。指臭味相投的人结合在一起。act in collusion with, wallow together in the mire, rise from the ranks, band together