In To Kill a Mockingbird, Chuck Little says, "Yessum," while talking about a head louse. What does this mean and what is the origin?

Little Chuck Little is one of the minor characters whom
the reader can really enjoy--even though his character is only in just a few pages of
the whole text.

Chuck is one of the students who fits oddly
into the social classes.  He is poor like the Cunninghams, and he doesn't know "where
his next meal was coming from." But unlike the Ewell family, he has manners.  He is very
polite to Miss Caroline, offering her help back to the front of the room after her
"cootie" incident, and he even offers to get her a cool glass of water.  With these
manners, he also addresses her as "Ma'am."  So when he says, "Yessum, he's alive" he is
saying Yes, Ma'am.  This spelling just gives the story its own southern slang style.  As
the chapters progress, there will be plenty of more examples to give the reader auditory
imagery with both slang and allusions to the past.  Tom Robinson will show his own
personality through his slang/speech as well.