Monkey Wrench

From Adjustable

Monkey Wrench What monkey wrenches have to do with monkeys is unknown. The term for a wrench with an adjustable jaw dates to the early 19th century and is originally British, although now is chiefly North American in usage. It has been suggested that the monkey is an alteration (folk etymology) of the inventor's name, but this explanation lacks supporting evidence.

The phrase to throw a monkey wrench into the machinery dates to 1918, although the metaphorical sense of throwing a monkey wrench, meaning an obstacle or hindrance, is a bit older. On 30 July 1907 the Chicago Tribune published the following:

It should look to them as if he were throwing a monkeywrench into the only market by visiting that Cincinnati circus upon the devoted heads of Kentucky's best customers. The British version of this phrase, to throw a spanner into the works, dates to 1934.

(Source: Oxford English Dictionary Online)

Charles Moncky invented the monkey wrench around 1858.