the.com/monkey wrench

means A wrench with an adjustable jaw for gripping nuts and bolts of different sizes — and, figuratively, a sudden snag thrown into a plan ('throw a monkey wrench into it').

from The tool itself is American, from the 1800s, but no one is fully certain why 'monkey' got attached to it — one popular story credits an inventor named Monk, but this is likely folk etymology rather than fact. More plausibly, 'monkey' was simply a go-to word in English slang for mechanical gadgets and makeshift tools, the way we might now say 'gizmo.' The figurative sense — tossing a monkey wrench into the works — followed naturally once people imagined the chaos of literally dropping a heavy adjustable wrench into moving machinery.