In the Adventures of Huckleberry Finn, what does Buck say when Huck asks him how the feud started?

The answer you are looking for can be found in Chapter
Eighteen of this excellent novel. Be aware that through these two rival families Twain
is satirising the nature of feuds and the concept of honour in the South. Note how Buck
explains what a feud is to the curious Huck:

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"Well," says Buck, "a feud is this way: A man
has a quarrel with another man, and kills him; then that other man's brother kills him;
then the other brothers, on both sides, goes fro one another; then the cousins chip
in--and by and by everybody's killed off, and there ain't no more feud. But it's kind of
slow, and takes a long
time."

It is obvious that
this hilarious response focuses on the ridiculous nature of feuds, as families slowly
are eradicated as one by one members are killed and the remainder feel honour-bound to
kill in response, triggering a never-ending cycle of violence and sadness. When Huck
asks Buck what the original reason for this feud was, Buck admits that he does not know
why such a longstanding feud exists between these two families. Thus Twain satirises
such feuds, which are often maintained long after the original wrong was committed. Even
though nobody can remember this original wrong, the reason for so much bloodshed, the
Grangerfords and the Shepherdsons are shown to be more in love with the feud and
violence than with any possibility of peace.