British -> American "Translation", Please

[Jim Flanagan]

I'm confused by the British use of the adjective "bemused,"  
as it is used to describe a person's facial expression or 
overall attitude. I suspect that most Americans (at least those 
without a degree in English) would take this to mean "slightly 
amused," or perhaps "self-satisfied."

However, after encountering forms of the word several times in the HP 
books, as well as in CS Lewis, it appears, contextually, to 
mean a "passive" or "spaced-out" expression or attitude.

I don't have the OED-online, so can someone clue me in on the British 
understanding of "bemused?"

-Jim