The monkey wrench is an adjustable wrench not much used today. Its use has generally been replaced by the Crescent®, or adjustable–end, wrench.

Concerning the origin of its name, this from William Rogers, The Progressive Machinist, Theo. Audel & Company, New York, 1903:

In his interesting article upon the genesis of machine design, Mr. W.H. Sargent spoke of the slide which moves up and down in the handle of a monkey wrench as resembling a toy monkey, and thereby drew an analogy. To this Mr. H.E. Madden writes: "The wrench is not named from this, neither is it so called because it is a handy thing to 'monkey' with. The right name is 'Moncky.' Charles Moncky, the inventor of it, sold his patent for $2,000, and invested the money in a house in Williamsburg, Kings County, N.Y., where he afterward lived."

This design served as the basis for all later adjustable end wrenches, including pipe wrenchs.

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Also used as a verb to mean sabotage. A literary example is The Monkey Wrench Gang, a novel by Edward Abbey from 1975.