What is "underhook"

An underhook is a clinch hold that is used in grappling to control the opponent. It is performed from any direction by putting an arm under the opponent's arm, and holding the opponent's midsection or upper body. Having an underhook with one arm is called a single underhook, while having underhooks with both arms is known as double underhooks. The typical response to an underhook is to try to break it, or to establish an overhook.