When you “throw a monkey wrench into” something, you’re sabotaging or frustrating a project or plans. For example, you might say, “My boss threw a monkey wrench into my plans for going away for the weekend when he said I’d have to work Saturday.”
There is a theory that the “monkey” in monkey wrench is a misspelling of the inventor’s name, a Charles Moncky of Baltimore who was supposed to have invented it 1858. But this explanation doesn’t work because it was the British who invented the term and it predates Moncky by many years.
It’s not known what monkey wrenches have to do with monkeys. The word “monkey” has been used for various devices, from cannons to pile-drivers. Perhaps because monkeys could learn how to use such tools? Perhaps it’s because a wrench has an adjustable jaw, which makes it handy for manipulating a variety of objects and that’s something monkeys can do, too.
Throwing a monkey wrench inside machinery, thus damaging or destroying it, is industrial sabotage, and the use of the phrase in this way dates from the early 1900s or before. The term is also used metaphorically, as in the following example. From the American Economics Review 1918: “Mr. A. Paladini, one of the larger wholesale dealers…threw a monkey wrench into the machinery of proposed fish distribution.”
The British version of this phrase is “throw a spanner into the works.” From P.G. Wodehouse’s Right Ho, Jeeves, published in 1934: “He should have had sense enough to see that he was throwing a spanner into the works.”
The phrase conjures up vivid images for me. Monkey wrenches come in various sizes and I can “see” a big one being thrown into a computer tower, for example. Or a monitor. Or a bowl of half-set Jello. Ouch!