literally "to borrow a path to conquer Guo"; to use a neighbor as a stepping stone to destroy a third party, then turn and conquer the neighbor; to connive with someone to damage a third party and then turn on the partner

Borrowing the resources or passage of an ally to attack a common enemy, then turning on the ally; an idiom based on a story from the Spring and Autumn Period where the State of Jin asked for passage through the State of Yu to attack the State of Guo, only to conquer Yu on the way back