A monkey wrench was a hand tool with adjustable jaws for turning nuts of varying sizes.
In 2267, James T. Kirk described sabotaging a disintegration station on Eminiar VII as "throwing a monkey wrench into the machinery." (TOS: "A Taste of Armageddon")
In a vision experienced by B'Elanna Torres in 2376, the Klingon equivalent of a monkey wrench was found lodged in the port nacelle of Torres' shuttlecraft. (VOY: "Barge of the Dead")