In chapter 25 of Harper Lee's To Kill a Mockingbird we find Jem and Scout discussing a recent threat made by Bob Ewell. Jem tries to calm down Scout from becoming worried by telling her that
Mr. Ewell was more hot gas than anything
This is a colloquial phrase that indicates how Mr. Ewell is just a guy who blows up and does not much more about it. He spits out mean things about Tom Robinson, about Atticus, and about anyone that h disagrees with. Yet, he would not be courageous enough to come near Atticus nor the children. That is, at least, what Jem tries to tell Scout.
However, we can see how this hot-head nature of Bob Ewell transfers to the press in statement that he makes once he finds out that Tom Robinson has died, even though unfairly:
one down and about two more to go
Therefore, the phrase makes allusion to the low and coward character of Bob Ewell.