The monkey wrench 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 Its use has generally been replaced by the adjustable-end wrench, which has a compact head and so is more easily used in confined places.The term "monkey wrench" is also used colloquially (and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 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.H