What does scout mean when she says that jem is counting his chickens?

It means that Jem is thinking that Atticus had won the case before the jury had come back with the verdict. I think that is the part where Jem is pounding on the rail saying "we got him, we got him" when Bob Ewell writes his name. Scout dosn't know that Tom is crippled and Bob Ewell is a lefty so he would have beaten up Mayella. Scout dosn't know any of this. She just thinks that Jem is assuming things.