In this reprimand spoken by Calpurnia to Jem, Harper Lee has Calpurnia speak in dialect using slang. The traditionally grammatically correct construction would be "If Mr. Finch doesn't wear you out." The slang expression "wear you out" refers to giving someone a beating or spanking. The meaning behind the expression is a hyperbole, or an exaggeration made for effect. The concept is of someone spanking or beating a child so long and so hard that the seat of the child's pants begins to wear thin or threadbare--the very clothing would wear out because the beating was of such long duration. (Obviously, that is an exaggerated action.)

The phrase is one of Calpurnia's "rusty threats." Neither Mr. Finch nor Calpurnia have any intention of actually spanking or beating Jem.

In this one line, Harper Lee incorporates dialect, slang, and hyperbole. 

I believe this to contain a colloquialism, or an informal phrase from a certain geographic region.  In this case, "wear you out" means a spanking, and was (possibly still is) used commonly in the southern United States.

Check the link below for more information on this literary term, as well as other important terms that will help your study of other literary works.  Good luck!

Great sentence with two literary devices!  This first is regional dialect.  Calpurnia's phrase "wear you out" is a Southern phrase referring to spanking.  Using regional dialect is one way in which Lee brings authenticity to her tale and develops the character of the town.  Notice also the use of the incorrect verb form "don't" in place of "doesn't".  Also a regional form.

The other device is a euphemism.  The term 'wear you out' is a ever slightly more gentle way of saying "If Mr. Finch don't spank you".