the.com/crank

means A person with a fixed, often obsessive opinion that strikes everyone else as unreasonable — until, sometimes, it doesn't.

from Crank started as the literal bent-handle device you turn to make a machine run — a crankshaft part, from Old English 'cranc,' related to 'crincan' meaning to bend or yield. By the 1800s the mechanical sense bent into a human one: a 'crank' became someone whose mind seemed to turn on its own eccentric axis, fixated and hard to budge, cranking out the same odd idea again and again. The image is exact — a crank is a mechanism that goes round and round in a fixed, narrow loop, and so, the slang insists, does a certain kind of stubborn believer.