That much overworked word “crank” gained universal vogue in connection with Guiteau’s assassination of President Garfield, but it was long before that applied by the late Don Piatt, who claimed to be its inventor, to Horace Greeley –- the purpose of it being to liken the famous editor to the crank of a hand organ,which is forever grinding out the same old tunes. The word, as we have now come to apply it, means much more and worse; it implies a condition of mind verging upon insanity, and this has given rise to the erroneous notion that it has its origin in the German word “krank.”The hand organ association (allegedly) proposed by Don (or Donn) Piatt, a Civil War veteran and journalist, apparently didn't become part of the word's official history. The OED says the colloquial crank -- "a person with a mental twist ... esp. one who is enthusiastically possessed by a particular crotchet or hobby; an eccentric, a monomaniac" -- dates to 1833: "Uncle Sam's ‘Old Mother Bank’ Is managed by a foreign crank." Since Piatt was born in 1819, he would probably have known this use of crank before he came up with the organ-grinding metaphor.
And though our crank is not derived from German krank, the words do share an ancestry. Crank comes from a rare Old English verb, says the OED, meaning "to fall in battle, of which the primitive meaning appears to have been ‘to draw oneself together in a bent form, to contract oneself stiffly, curl up.’ There are "numerous derivatives" in various languages: "English crank belongs to the literal sense-group, with the primary notion of something bent together or crooked; German and Dutch krank adj. ‘sick’, formerly ‘weak, slight, small,’ shows the figurative development."