An almost-synonym for broner. The difference between a chumby and a full-on broner is the degree of erection. See chubby and boner for more detail. If definition is still not understood, see pun.
I was playing pong with my buddies the other day when I felt a chumby coming on.
Maerri: "Hey chumbies, check this out - when you sing in the chumby, your acoustics reverberate throughout the chumby and it's keeping me awake which makes me feel real chumby. Would you mind keeping it chumby?"
Culver: "Whoah, put your chumby on a leash."