Monkey Wrench Meaning

(idiomatic) An unexpected or troublesome problem, obstacle or dilemma.

Example: A wrench (spanner) with an adjustable jaw.
1903, Shop Talk, Volumes 1-3, page 21,
  Did you ever stop to think whether or not you knew how a monkey wrench should be used?
1948, George M. Warren, Simple Plumbing Repairs in the Home, Farmers' Bulletin, Issue 1460, page 2,
  With a monkey wrench on the hexagonal part of the stuffing box unscrew the stuffing box from the body of the faucet.
2010, A. M. Murray, The Secret of the Serpent Mound, unnumbered page,
  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.
  Having a baby will add a monkey wrench to her routines.
2003, Robert D. Keppel, William J. Birnes, The Psychology of Serial Killer Investigations: The Grisly Business Unit, page 130,
  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.
2005, Julia E. Richards, R. Scott Hawley, Catherine A. Mori, The Human Genome: A User's Guide, page 159,
  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.
2007, Carol Hoenig, The Author's Guide to Planning Book Events, page 59,
  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.
2009, Ken Reamy, The Balaam Surrogate, page 210,
  Girard had programmed some electronic "monkey wrenches" into the control panel that with the stroke of a key, would render the device inoperable.