Etymology Already in use, and not unfamiliar among mechanics, by 1832; see discussion and references at Monkey wrench § Etymology and history.
1
A wrench (spanner) with an adjustable jaw.
Did you ever stop to think whether or not you knew how a monkey wrench should be used?1903, Shop Talk, volumes 1-3, page 21:
With a monkey wrench on the hexagonal part of the stuffing box unscrew the stuffing box from the body of the faucet.1948, George M. Warren, Simple Plumbing Repairs in the Home: Farmers' Bulletin, number 1460, page 2:
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There were two monkey wrenches for that, sitting in a bucket. Lisa held one wrench and I applied the other and in a few minutes we had removed the motor head and set it on the ground.2010, A. M. Murray, The Secret of the Serpent Mound, unnumbered page:
2
Figuratively An unexpected or troublesome problem, obstacle or dilemma.
Professor Miller's careful research has thrown a monkey-wrench into the Einstein wheel, although it has far from discouraged the enthusiastic exponents of Einsteinism.1926, New Outlook, volume 142, page 58:
Meanwhile the killer, either knowing for sure or by instinct that the police have incorrectly profiled him, throws monkey wrenches into the task force investigation by changing his modus operandi (MO) for no other reason than to trick the police.2003, Robert D. Keppel, William J. Birnes, The Psychology of Serial Killer Investigations: The Grisly Business Unit, page 130:
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It has been suggested that myocilin missense mutations that cause severe disease are monkey wrenches, causing disease through actively causing a problem rather than through a lack of the gene product or function.2005, Julia E. Richards, R. Scott Hawley, Catherine A. Mori, The Human Genome: A User's Guide, page 159:
Therefore, even if the author is a celebrity, something may throw a monkey wrench in an event's success. It is here where I must mention the monkey wrench that dramatically hurt events I scheduled during the period of 1996–2001.2007, Carol Hoenig, The Author's Guide to Planning Book Events, page 59:
Girard had programmed some electronic "monkey wrenches" into the control panel that with the stroke of a key, would render the device inoperable.2009, Ken Reamy, The Balaam Surrogate, page 210:
Etymology Already in use, and not unfamiliar among mechanics, by 1832; see discussion and references at Monkey wrench § Etymology and history.
1
To apply a monkey wrench, e.g. to tighten or loosen nuts.
2
Idiomatic To introduce problems, to create unexpected trouble, to sabotage.
I think it’s possible that [US Vice President J.D.] Vance didn’t intend to monkey wrench [the meeting with US President Donald Trump and Ukrainian President Volodymyr Zelensky].2025 February 28, Jonah Goldberg, “Dishonor and Incompetence in the Oval Office”, in The Dispatch: