Lavations is a rather old-fashioned and quaint word, and To Kill a Mockingbird by Harper Lee is set in a time and place where such a word would regularly have been spoken.
In chapter eighteen of the novel, the citizens of the town had a very good reason to make themselves more presentable than usual. Tom Robinson's trial is about to begin, and it has become quite a spectacle. Scout, Jem, and Dill have come to watch, despite Atticus's request that they stay home. Scout it the narrator of the novel, and she makes this observation:
In Maycomb County, it was easy to tell when someone bathed regularly, as opposed to yearly lavations: Mr. Ewell had a scalded look; as if an overnight soaking had deprived him of protective layers of dirt, his skin appeared to be sensitive to the elements.
In context, lavations seems to have something to do with bathing and keeping clean, and that is the correct definition. Lavation is the act of bathing or getting oneself clean, so Scout's observation is that Bob Ewell is not used to regular lavations (bathing), though he did seem to make an effort for his day in court.