Cracker

Some people believe the origin of the anti-white epithet "cracker", comes from the times of american slavery where white slave owners would "crack the whip" over their black slaves. This is a misconception. The exact etymology is still up to debate, so pretending to be so sure of its origin is still a mistake. The following is the most probable origin.
The predecessor of the word crack was the middle english word crā̆k, this word was adopted into Gaelic as the word craic meaning a "loud conversation, bragging talk", the derivative of this word was used in Shakespearean time meaning "entertaining conversation" (conected with "to crack a joke"). A loud braggart would be described as a cracker.
Later on in America the word cracker would describe poor Celtic immigrants, as they were seen as "unruly and ill-mannered". Later on the term would be used to describe poor white people in general and there would even be a connection to whip cracking, but only in connection to herding animals, since the people that were called crackers were too poor to possibly own any slaves. (although the connection of cracker to slavery is still possible)
Source is from Wikipedia :)