Someone who you think is great and who thinks you're great too. Comes from the word bumming which does not always mean the act of Anal Love Making but liking a person a lot. E.g "You're ALWAYS bumming her!"
Chumbums are people you bum (in the context as above) and who bum you!!
Person 1:"Hey Chumbum"
Person 2:"Haha! Hey what you been up to?"
Person 1:"Nothing much!"
Person 2:"Wanna come over my house"
Person 3:"You two stop bumming eachother!!"