Stolen Wheelchair

Listen here, you knucklehead. Your brain is smoother than a baby's bottom. The joke is funny because it's dark humor. The narrator's girlfriend dumped him, and then, like the petty little gremlin he is, he took her wheelchair. Then he makes a pun by saying she came "crawling" back, because she literally has to crawl without her wheelchair. It's funny because he is a bad person.