All great writers give careful thought to naming their characters, though how Mark Twain decided upon Huckleberry Finn is not known for certain. What is known is that Twain had to rename a character in his book The Guilded Age from Colonel George Sellers to Mulberry Sellers in order to avoid a libel suit. The name Mulberry was probably in the back of Twain’s mind when he came up with Huckleberry. The actual huckleberry Mark Twain only knew by word of mouth until moving into his home in Hartford in 1868. The word “huckleberry” comes from the English “hurtleberry,” which it resembles. The first instance of the word comes from 17th century American botanicals.
It has been suggested that the word Huckleberry suggested to Twain a lowly person of rustic origins, much like the plant that produces small fruit, and resists cultivation. Twain himself says that he especially liked the shortened “Huck” and that he obtained the last name, Finn, from a real life Irishman and alcoholic who lived close to Twain in Hannibal. The character Huckleberry Finn was based a boyhood friend of Twain’s, Tom Blankenship, who, like Huck, was the son of a town drunk.
It's a mix of the actual last name of the boy who the character was based on with a term or phrase that Twain thought would emphasize his lower class origins. He also said that he liked the sound of the abbreviated version: Huck Finn.
Ha, well, I know! You may not beleive me, but it's been a family story and it's been genetically proven in my family. Well, my great-great-great grandfather's name was Benjimn Finn. He grew up in Mississippi and was childhood friends with Samuel Clemens. But my greatgreatgreatgrandfather's nickname was Huckleberry Finn because of his insane love for huckleberries. It's all true, weither you beleive it or not. Haha, ain't it cool?