I think the actual origins on this are still unclear, but the "shaking of the stick part" seems to stem from a gesture of threatening with an actual stick, with the original meaning loosely being "more than one can threaten." These days, it's used to indicate a large number of something, i.e. "more than you can shake a stick at" - which would imply that the actual number is more intimidating than the threatening stick shaking behavior. I know, that sounds wacky. Here are some additional links:

http://www.worldwidewords.org/qa/qa-sha2.htm

http://www.yaelf.com/aueFAQ/mifmrthnycnshkstck.shtml

I've always thought the point was just that it requires so little effort to shake a stick at something. I.e. "more than you can shake a stick at" = so many that you can't even do that. I think this interpretation applies well to the archaic examples cited in the first worldwidewords link, e.g. "nothing worth shaking a stick at" = nothing deserving of even the most minor consideration. I don't really see how the threatening nature of a certain type of stick-shaking is alluded to by any of the citations.

I think it means to threaten in the European/African dialect but to do something easily in American dialect

@Mil zack; what do you mean by European/African dialect and concomitantly, American dialect? Those terms mean nothing to me.

Languages evolve, so maybe when the Europeans came to America, the term for threatened somehow changed to to easily do something. This can be because of a war, event, craze, or anything else that affects the people who use the phrase. These differences are known as dialects