A monkey wrench is a large adjustable wrench with smooth jaws that can grip pipes and nuts of different sizes. You turn a screw mechanism to open or close the jaws, letting one tool work on many different sizes of bolts. Plumbers and mechanics used monkey wrenches constantly before more specialized tools became common.

Today, people rarely use actual monkey wrenches since pipe wrenches and adjustable crescent wrenches work better for most jobs. But the phrase lives on in a common expression: to throw a monkey wrench into something means to cause a disruption that stops progress or creates problems. If your teacher plans a field trip but then the bus breaks down, that breakdown throws a monkey wrench into the plans. When you're working smoothly on a group project and someone loses the supplies, they've thrown a monkey wrench into your work. The image is perfect: imagine a spinning machine suddenly jamming when someone tosses a heavy wrench into its gears.