old style of wrench with a jaw adjustable by a screw mechanism on the handle, 1841, from monkey (n.) + wrench (n.). Monkey was used in 19c. especially by sailors, as a modifier for various types of small equipment made for specific work (monkey-block, monkey-boat, monkey-spar, etc.), and the same notion probably is behind the name of the tool. The figurative sense of "something that obstructs operations" is from the notion of one getting jammed in the gears of machinery (compare English spanner in the works).