Well I (a Brit) have only ever understood it to mean 'not sufficiently impressed to know what to say'; in a sort of bemusement or indecision as to one's reacti

Well I (a Brit) have only ever understood it to mean 'not sufficiently impressed to know what to say'; in a sort of bemusement or indecision as to one's reaction. A sort of confusion as to how unimpressed one is. The Collins dictionary I have to hand doesn't capture that nuance though. I actively try to avoid USAmericanisms, except for "y'all". Go figure ;o) [That last phrase is a USAmericanism, used for amusing intent.]