the.com/dry gulch

means To ambush someone by waiting in hiding, especially to attack or betray them when they least expect it.

from Straight out of the American West: a 'gulch' is a dry ravine or gully carved by seasonal runoff, and 'dry-gulch' emerged as cowboy slang for ambushing someone there — luring or catching them in a desolate ravine where no one would hear or see the attack. By the early 1900s it had leaped from literal canyon ambushes to a general verb for any sneak attack or betrayal, especially a political or business double-cross.