Wordorigins.org

What monkey wrenches have to do with monkeys is unknown. The term monkey has been used for a variety of devices, from cannons to pile-drivers, and it seems likely that the wrench’s name is related to this usage, but exactly how is uncertain.

This term for a wrench with an adjustable jaw dates to the early 19th century and is originally British, although now it is chiefly North American in usage. From a citation believed to be from 1807 that appears in E.S. Dane’s Peter Stubs & Lancashire Hand Tool Industry:

Fleetwood, Richard...Parr, Rainford. Screw plates, lathes, clock engines...monkey wrenches, taps.