The American-English phrase to throw a monkey wrench into means to stop or obstruct by direct interference—British-English synonym: to throw a spanner in(to) the works.
I have discovered that the original image:
– was of throwing a monkey wrench into the cylinder of a threshing machine, i.e. a power-driven machine for separating grain or other seed from the straw or husk;
– appeared in the late 19th century;
– exclusively referred to political situations.
The earliest instance of this image that I have found is from a dispatch from Washington, D.C., about a debate in the House of Representatives, published in the San Francisco Chronicle (San Francisco, California) of Wednesday 6th July 1892:
Bland’s action in insisting upon amending the Stewart bill has been severely criticised. He is charged with occupying the position of the man who threw a monkey-wrench into a threshing machine because he was not allowed to feed it. The trouble with Bland seems to be that it is Stewart’s bill and not his. He wants all the fame, even if he jeopardizes the cause in which he proposes to lead.
The second-earliest instance of this image that I have found is from The Los Angeles Times (Los Angeles, California) of Sunday 12th August 1894—the term threshing machine itself is not used, but the word hopper is; however, the use of the verb grind might indicate that the reference is to a grinding mill:
Santa Ana, Aug. 11.—(Special Correspondence.) The “unterrified”—the Democracy of Orange county¹—met in Neill’s Hall at 10:40 a.m. today and proceeded to select delegates to the State convention, to be held in San Francisco, August 21.
The hosts assembled again at 1:30 o’clock, and from the expressions worn by a few of the more anxious delegates it was seen that the committee had been having a taste of the trouble that was brewing. The old machine, however, was started with a full hopper and had only ground out the Committee on Resolutions when Mr. Hargrave threw a monkey-wrench into the cylinder by offering an amendment to the report to the effect that the convention disapprove and disavow the action of the County Central Committee in passing the set of resolutions spoken of in The Times a few days ago.
(¹ on Orange County, New York, see origin of ‘Indian summer’ and French ‘l’été sauvage’)
The image of throwing a monkey wrench into the cylinder of a thrashing machine also appeared in the last paragraph of Teddy’s Night in Town, published in The Davenport Times (Davenport, Iowa) of Saturday 6th October 1900; this article was about the visit of Theodore Roosevelt (1858-1919), who had been nominated for Vice President² at the 1900 Republican National Convention:
Just one thought with which to close. The Republican party has brought prosperity and has brought the nation from a debtor to a creditor nation.
As the farmer when he gets competent men to run his threshing machine that has been running bad for four seasons and has a dry season in which to thresh and the grain is grading well, rejoices, so ought we to rejoice. Don’t, then, on the 6th of November, go into the election booth and throw a monkey wrench into the cylinder.
(² Theodore Roosevelt became the 26th President of the United States after President William McKinley was assassinated.)
According to the Democrat and Chronicle (Rochester, New York) of Friday 19th October 1900, Leslie Mortier Shaw (1848-1932), Governor of Iowa, also used the image during the presidential campaign:
Don’t Fool With the Machinery.
The Republicans of Iowa have discovered during the campaign that in Governor Shaw they have one of the most effective and convincing stump speakers in the field. He is especially popular and telling with rural audiences, to whom his homely and apt illustrations always appeal with force. They are saying out in the state that Shaw always talks good horse sense and he never talks over people’s heads. In a recent speech at a farmers’ meeting up in the northwest corner of the state he used the following illustration:
“You get up early these fall mornings; fog and mist and drizzle hang over everything; it is cold, belts slip, shocks are damp, men are cross, the engine don’t steam, it seems as if you would never get started. Presently the sun rises, the mist vanishes, things warm up, the men are cheerful, the horses prick up their ears, the machine hums, the golden grain fairly boils into the measure, the men on the stack begin a song, and a good day’s work is in prospect, when just then some fool drops a monkey-wrench into the cylinder! My friends, prosperity has just begun to work nicely; don’t, for mercy’s sake, throw a monkey-wrench into the thrashing machine.”