Comment by ofalkaed

>Surely "howdy" derives from "how do you do?" and not "hello."

It seems most likely but the OED (at least in the second edition) says:

>[Note. The conjectured derivation from the phrase how d'ye? is impossible, since the Sc. form would then have been (huːdɪ). On the analogy of Sc. gowdie = goldy, howdy might go back to holdie, an appellative (like brownie, etc.) from hold, friendly, benevolent, kind: cf. F. sage-femme.]

But the OED has many oddities regarding American vernacular and I personally take it with a grain of salt when it comes to this area. It's only definition for "howdy" is as an alternative spelling for "howdie," a midwife and ignores the common US idea of it being "how do you do," but it does include a "see how d'ye" where it includes "howdy" and various other spellings for the sentiment of "how do you do."