to me "bimbo" means an uncomplimentary word for a heavy set woman, but I've also heard it mentioned (as American slang) for a heavyset blonde woman who acts stupid..
"Bimbo is a term that emerged in popular English language usage in the late 20th century to describe a stupid, pliable woman. (Its occurs as early as 1929: in the silent film Desert Nights, a title describes a cheap female crook as a bimbo.) Because it derives from Italian words of masculine gender, its first usage in English was for stupid men; it now is understood to mean a woman unless modified as male bimbo, himbo, or mimbo. The word bimbo is sometimes interpreted as the backronym "Body Impressive, Brain Optional".
A bimbo isn't necessarily highly sexually attractive. Being a bimbo is a state of mind, and reflects a person who exaggerates the effort and value put into her physical attractiveness. She is often perceived to be shallowly focusing on her physical appearance and neglecting or even willfully stifling the development of other parts of her personality."