> There may be some irony in this. Since 'krank' in German > basically means 'sick', and 'Er ist krank [im Kopf]' might be used to > mean 'He is sick [in the head]', I would guess that our use > of the word 'crank' to describe someone who is basically a > nut of some kind is derived from the German.
Quite possible IMO.
"Crank" in our sense seems to be have originated in the 20th Century. Maybe it derives from talk of "crank telephone calls" or "crank letters". Math cranks do have a tendency to pester people with communications.
Off topic, remember the great news correspondent Walter Cronkite? Adapted from Krankheit, I'll bet.