In this mental_floss article, Neatorama's very own David Israel tells us the origin of some of the most common nicknames in the West:
The name Henry dates back to medieval England. (Curiously, at that time, Hank was a diminutive for John.) So how do we get Hank from Henry? Well, one theory says that Hendrick is the Dutch form of the English name Henry. Henk is the diminutive form of Hendrick, ergo, Hank from Henk. Hanks were hugely popular here in the States for many decades, though by the early 90s it no longer appeared in the top 1,000 names for baby boys.