refers to one who cannot exist in the absence of coffee. he or she is then "a slave to the bean"
Jill: I can't go anywhere until I have my morning coffee. No...I don't care how badly you're bleeding from the ears, Jack.
Jack: aaaaaaaaaarggh! i'm dying and you won't drive me to the hospital until you've had some coffee?!?! Jill, you're such a slave to the bean.