The monkey wrench or in the UK "Adjustable Spanner" is an adjustable wrench, which is rarely used today. Its use has generally been replaced by the adjustable-end wrench, which has a compact head and so is more easily used in confined places.
The term "monkey wrench" is also used colloquially (and inaccurately) to refer to the pipe wrench.
Concerning the origin of its name, this story has been repeated in several publications in the late nineteenth and early twentieth centuries:
That handy tool, the "monkey-wrench", is not so named because it is a handy thing to monkey with, or for any kindred reason. "Monkey" is not its name at all, but "Moncky." Charles Moncky, the inventor of it, sold his patent for $2000, and invested the money in a house in Williamsburg, Kings County, where he now lives.
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.
However, this story has been refuted by historians, and there is evidence that screw-adjustable wrenches and even the term monkey wrench were used long before, at least from about 1840.