"Cham is the only thing to screw one up when one is down a peg"

In this quote, Anthony Trollope is suggesting that alcohol, particularly champagne (also called "cham"), is the only thing that can intensify somebody's scenario when they are currently feeling low. The expression "down a peg" refers to feeling humbled or torn down a level. Trollope is implying that when someone is already feeling down, turning to alcohol can only make things worse. This could be translated as a warning versus using alcohol as a coping system or a way to escape from one's problems. Trollope's words function as a tip to be mindful of our actions and options, especially when we are feeling susceptible.