American Machinist, 1880 Mar,Vol3 Pg9 gives this somewhat diferent explanation: (slightly garbled OCR)
"Would yon be Bo kind as to give me the/lerivation of the term "monkey" as applied to a wrench?
A.—The term monkey was first applied to a weight used as a hammer, as in the pile-driver, and is known by that name at the present time. When the weight is used, as in a drop hammer, to form or indent metals, it is also known as a " monkey hammer." The term " monkey" is also applied to a pump, which is the sailor's name for the sucking straw introduced at a gimlet hole in a wine cask. It is as old as Xenophon, who describes this mode of pilfering from the wine jars of Armenia. This style qf pump will probably remain in use much longer without improvement. The term "monkey" as applied to a wrench signifies hammer, as a monkey wrench is part hammer and part wrench, forming a very useful combination. "
As is typical, no references are cited :(
At least it doesn't require mangling some poor fellows name ;P