An awesome friend. Someone you can make fun of without them pulling out your hair. Preferably someone who goes out of their way to fix your life. If you don't have a chum, your life friggin sucks.
P.S. chum is also dog food :P
"Make me an offer I can't refuse, chum."
"Lit, chum. Real lit."
You: "Ellie will never like me!"
Chum: "HEY ELLIE!"
You: "Wtf are you doing?!"
Chum: "Ellie, look into my eyes and tell me you don't like my friend."
Ellie: "Why would I do that?"
Chum: "'Cuz you can go out with him."
Ellie: "I'll think about it"