An abscess can refer to a pocket of pus that forms around the root of an infected tooth. "Abscessed by it" sounds like "obsessed by it" which means to be continuously preoccupied with something. The joke is referring to someone who keeps thinking about the pain in their tooth, and the word "abscess" was chosen as it closely matches the sound of the word "obsess" and can relate to why their tooth hurts.
I was fixated on the pain in my bad tooth . I was abscessed by it .