monkey wrench in the works, put/throw a

Fig. to cause problems for someone's plans. I don't want to throw a monkey wrench in the works, but have you checked your plans with a lawyer? When John suddenly refused to help us, he really threw a monkey wrench in the works.

Sabotage an operation or plan. The monkey wrench, called an “adjustable spanner” in Britain, appears to have reminded someone of a monkey’s jaws, which loosely resemble the sliding jaws of this very useful tool. This name was acquired about the middle of the nineteenth century. It was not until the early twentieth century that it became associated with sabotage. This suggestion first appeared in print in 1920 in Philander Johnson’s story, Shooting Stars: “Don’t throw a monkey-wrench into the machinery!” The locution not only caught on in America but was adopted in Britain as well, although in the form of throw a spanner in the works.